Vibration at 70 mph and checking diff oil levels

[74 challenge]

So it seems I have developed a pretty noticable vibration at 70 mph (my usual cruising speed) what are some areas I should chase first to see what it is?

also how do I check/ change diff fluid in a 74 challenger with highway gears in an 8 3/4 rear end? Since I have owned the car I have never done it and I can be pretty certain the previous owner didn't even know what a Diff is.

a904, stock driveshaft, highway rear gear....pretty base model.

[Jamiez]

8 3/4 was never designed to be serviced really well.  I seem to remember needing to suction the old oil out and then just filling it until it came out the hole.  The original plan was that unless you got the axle underwater it would never need filling but they probably did not predict a 40 year lifetime

[jhaag]

on the passenger side of the diff there is a plug just above the flat reinforcement casting that is parallel to the ground. usually takes a 3/8 drive ratchet to remove. over the years slow leaks may have your level down. you should be able to stick a finger in and touch the fluid. ideally it would just trickle out when you remove the plug, with the car level or close to it. as for the vibration, need more info. could be tires, u-joints, wheel bearings, trans, loose bolts, and on and on...

[DocMel]

This is a classic sign of the dif needing rebuilding. 

Before you look at wheel balancing, tourque converters, drive shaft balance, etc, etc, disconnect your drive shaft at the rear dif yoke

If you can wiggle the yoke itself up/down,left right, you are looking at a rear end rebuild  Ill bet $$$ thats where your issue is 

Remember, 40 plus year old rear ends are about worn out by now, and rarely, if ever, got serviced by previous owners.  Lubing a wore out rear end will not help the prob

Ask me how I know!  Right at 68-70 MPH Id get that annoying vibration.  I checked EVERYTHING, to include replacing all wheel bearings, u joints, wheel balance, drive shaft balance, flywheel weights, etc   

It was the rear end:  When taken apart, it was VERY, very  wore out

[Chryco Psycho]

you can use a suction gun [similar to a grease gun] to draw out the oil & replace 90% of the fluid , vibration could be bad U joints or bad driveline angle or worn out as suggested above

[74 challenge]

bad driveline angle could be the issue...my rear wheels are not centered in the wheel openings...the passenger side one sits about 1 inch furthur away then the drivers side...

Alrighty my plan of attack is:

1. check fluid
2. unbolt driveshaft and give the yoke a shake
3. Cry as the moths leave my wallet.

Whats a rear end rebuild go for? I guess this is the time I should ask about getting a 3.23 posi rear in there....

[DocMel]

Drive line angle is most likely the least of your cause. 

Check the easiest stuff first  and eliminate possibilities from there

Look at your u joints while someone turns the wheels.  Look for play, binding or clicks.  Move the shaft up and down.  There should be no movement at the joints   Be sure the u joint bolts are tight. 

Even if the joints, bolts check good OR bad, you still need to check the pinion yoke for play like I said : There could be more than 1 cause. 

Remove the drive shaft and check the pinion yoke for  play like I mentioned

Again, adding fluid will not reduce a vibration

I went thru this:  Id like to reduce you chasing the dogs tail like I did and save you some time

Rebuilding a rear end dif cost me $1100, and that was with me removing the entire axle and taking it to the vendor:  They pressure washed, wheel bearings were changed, rear end gears replaced, axle was painted, all new seals and it was filled with dif fluid when I got it back

Check the u joints , but also check the rear pinion yoke for play, even of the u joints check out bad or good : A bad rear end assembly will really put wear on u joints

Drive angle, to me at least, is a little over stated as a possible source of vibration.  I doubt wear on mounts have increased the angle so much that it is causing the prob.  Id place this low on the list of possible  causes

Look, many 4 wheelers  will put in a 2-4" lift kit with NO drive angle vib issues, and they run TWO driveshafts

Start with the easy stuff to check first.  Remember, whether the u joint checks out good OR bad, still check the rear pinion yoke like I said.  The vib could be caused by more than 1 issue

[74 challenge]

Drive line angle is most likely the least of your cause. 

Check the easiest stuff first  and eliminate possibilities from there

Look at your u joints while someone turns the wheels.  Look for play, binding or clicks.  Move the shaft up and down.  There should be no movement at the joints   Be sure the u joint bolts are tight. 

Even if the joints, bolts check good OR bad, you still need to check the pinion yoke for play like I said : There could be more than 1 cause. 

Remove the drive shaft and check the pinion yoke for  play like I mentioned

Again, adding fluid will not reduce a vibration

I went thru this:  Id like to reduce you chasing the dogs tail like I did and save you some time

Rebuilding a rear end dif cost me $1100, and that was with me removing the entire axle and taking it to the vendor:  They pressure washed, wheel bearings were changed, rear end gears replaced, axle was painted, all new seals and it was filled with dif fluid when I got it back

Check the u joints , but also check the rear pinion yoke for play, even of the u joints check out bad or good : A bad rear end assembly will really put wear on u joints

Drive angle, to me at least, is a little over stated as a possible source of vibration.  I doubt wear on mounts have increased the angle so much that it is causing the prob.  Id place this low on the list of possible  causes

Look, many 4 wheelers  will put in a 2-4" lift kit with NO drive angle vib issues, and they run TWO driveshafts

Start with the easy stuff to check first.  Remember, whether the u joint checks out good OR bad, still check the rear pinion yoke like I said.  The vib could be caused by more than 1 issue

your post fills me with uneasy feelings and sadess lol. I now hope it's a bad u joint...a rebuild sounds awfuly expensive.

To remove the driveshaft at the yoke is just the couple of bolts corerct? there is no science to it?

[Chryco Psycho]

correct 
if you raise the rear of the car & slide under the back you may not lose any trans fluid either
The whine is generally bad gears , it will usually only whine in one condition either under power load side or off the power / coasting usually this cannot be fixed without new brgs & gears which will run $600 or so & if you have a 489 carrier you need to replace the crush sleeve with a spacer &shims . You can get the housing cleaned & painted or powder coated & install new axle inner seals . Re assembly is simple just drop in the carrier with a new gasket & silicone install the brake backing plates slide in the axles tighten up the 9 nuts holding the axles in place set the end play & install the 10th nut & adjuster lock install the housing connect the brake lines etc .

[DocMel]

I know.....It a potentially expensive thing to think about   My main reason for replying is to keep you form going  round in circles like I did

Checking the rear dif yoke for play is easy:

1.  Take a bungee, or rope, and tie it around your driveshaft in an aera about 2/3 toward the rear dif, and tie it off on your exhaust (so the shaft wont fall down)

2.  Remove the 4 bolts that hold the shaft to the rear yoke  Take the 2 small c - shaped u joint bands off.  They may just fall off, or just pry them a little

3.  While holding the driveshaft, tap the driveshaft from the dif side toward the tranny to get it unseated from the yoke carrier.  The shaft will move toward the front and into the transmission enough to clear the yoke.  AS SOON AS IT CLEARS THE YOKE, you might want to wrap tape around the 2 bearing caps that were in the yoke.  That will keep the cap ends from coming off, and the internal bearings from spilling out 

If you are sure the drive shaft wont slide out of the tranny, let it go

Now grab the yoke and shake it top to bottom, left to right.  Any play, and its time to have the dif looked at .  Turn a wheel, and watch the yoke and listen   "Crunching", or movement in the yoke aside from a solid circular motion is another sign

Myself, I would also change  out all the ujoints (front and rear) if you dont know when the last time they were changed out

[74 challenge]

You guys are almost aming more scared to look at it.

IF it comes to needing a rebuild I might just oder a complete center chunk and put it in myself. Assuming my axels would work with this option, but I will get there when I get there first is the diagnosis.

[DocMel]

Replacing, or rebuilding the dif guts in a read end is WAY more involved that it seems.  Special tools and measurement tools are required.  Its not just matter of take the center out, replace it with a new one  heck, I could do it, but I dont have the specialty tools to do so, and the expense of them doesnt justify , to me at least, to buy them for one job

If you are driving your car now, and you do have a dif prob, it salvagable.   But have a pro do the work

[Chryco Psycho]

I agree that setting diffs up is not for amateurs & the tools are too costly just starting with the press to install or remove brgs alone. buying a complete center section is an option as well , I would deal with Doctor Diff & use a Tru Trac sure grip 

[ShelbyDogg]

Check your wheels and tires first. I had a bad belt in one of my BFGs that caused a bad shake.  I swapped a 4 tires with my other car and the vibration moved.

Sometimes it is a simple as a tire balance.  One Power Tour day, I stopped at a Walmart tire dept. I jacked up the car and rolled the tires in to them one at a time to balance them.

As soon as I hit the highway my vibration between 70 and 80 mph was gone.

[DocMel]

Do it anyway you want   

Again, I can only suggest before you thru checking out tires, all 4 of them, or mounting tires off another car, (not a bad idea) or balancing tires  (Just like I did, I did both), just check for play in the rear end like I mentioned:     If its in the rear end, you will reduce wasted and unrequired labor, $$$$    Ill say again, the rear end is the ONE thing no one really ever maintianed over the last 40 years, second to u joints

I will agree with balancing tires:  Im assuming they were/are balanced?