Ideas on whats causing a new vibration.

[7Dcuda]

Asking for suggestions on what may be causing a vibration that started out of nowhere.  70 Cuda/440/727 transmission/8.75 rearend.  Car has been running fine and all of the sudden out of no where, without any hint of any sort of problem, I get a bad vibration that occurs approximately right before it shifts into second gear then goes away once more speed/rpm is reached.  Vibration is so bad there for about 3-6 seconds that it can be felt all in the steering column/wheel.  Any suggestions from those of you who have experience with this type symptom?   Thanks in advance.     

[ksierens]

Your torque converter may be coming apart.  I had this happen on a GMC truck I had. Also make sure the bolts for the torque converter to the flex plate are tight.

[burdar]

Does it happen right at the shift point and then go away right after the shift...or does it continue for a short time?  I drove my ex bosses Mustang that shook really bad at the shift points but was gone right after the shift.  It turned out to be a bad plug wire.  We both thought it was a trans problem but it wasn't.

The first thing I would check though, is to make sure the torque convertor bolts are tight.   

[7Dcuda]

Thanks guys, I'll check those things out.  Anyone else have any additional ideas?

[tactransman]

Does it do the same vibration if you rev it up in neutral?

[7Dcuda]

Does it do the same vibration if you rev it up in neutral?

Not sure about in Neutral, but revving in Park everything seems/sounds/feels fine...no skips or misses or vibrations.  Therefore, I dont think the vibration is in the engine but more likely to be drivetrain or transmission.  Can the torque converter to flexplate mounting bolts be easily accessed/seen?     

[Aussie Challenger]

Careful about reving in park as the oil is not pumping/circulating, pumps only in neutral and in gear unless you have bought the modified selector spool.   

[femtnmax]

To test for other causes, try driving the car above the vibration speed, then coast back down with trany in neutral, engine idling.  This will check for tire balance, driveline issues.  Check each tire when lifted off the ground, rotate tire and look for belt separation...tire will have a bulged area as you spin it by hand.  Are the engine mounts and trany mount OK, not broken?  Jack front wheels off the ground, are front wheel bearings adjusted correctly, long shot but threw it in?   
All previous posts are good suggestions, so do all those for sure.  Maybe do compression check on the engine, though you said the engine revved fine no problem.

[femtnmax]

I forgot an easy one.  Rotate the tires front to back, do not change sides, keep LH on LH, etc.   If vibration changes or goes away its a tire balance problem.

[Moparal]

Maybe a few other check pointa

loose u joint straps
Loose pinion nut
bad rubber on trans mount
loose trans mount to member bolts
loose rear shackle bolts
weight feel off rim
loose u bolt
bad shock
loose shock bolt
u koint cup sliding or u joint going bad

just a few things you may check