I haven't read all the responses, so bear with me if you have already had this idea: I had the same EXACT symptoms. I tried every damn thing in the book, and rebuilt what I thought was EVERYTHING that could've caused the vibration . It was finally found in the rear dif: Heres' the deal. Our rear difs are basically about 43 years old. And its the ONE item on our cars that is basically neglected, even with basic maintanance like adding or changing oil in the rear dif. Ring, pinion, axle bearings, clutch pack, etc will be sloppy over 43 years, and in bad need of replacement.
We typically never go into the rear dif ourselves for a rebuild because it does take some special tools, etc, to set up all tolerances
If you have tried everything except the rear dif, you need to have it checked out. Take the car into a reputable dif shop, or just take the axle. It only takes about an hour to completely remove the rear axle
I believe that's where your vibe is. I had the same exact thing/signs, but at 70 MPH It would start there, then seem to dissapear at about 80 mph. That's the only speeds I would feel it but it sure was aggravating. It would be perfectly fine otherwise, at other speeds
Bite the bullet and have the dif checked
