I'm wondering if any ideas other than my next "guess". At 85 mph the shifter and the car starts to vibrate pretty good, seems to be from the back end only. I replaced tires thinking they were getting low anyway. (even rotated new ones to make sure it wasn't a rim) replaced U joints in the drive shaft too since they had nearly 200k on them.
I can jack up the rear off the ground and go to 120 mph on the speedo and it's smooth as silk running too. so it's only when on the ground. Panhard, tranny mount??? Suggested to get a Global West bar in back to fix it...... thoughts?

 

does it stop when you speed up or slow down? how about when you push the clutch in or put the brakes on?

 

U joints or out of balance driveshaft, bearings going bad in the diff, Tires not rated for the speed you are driving.

Honestly the first thing I would do it have my tires pressure balanced. This is where they balance your tires while putting pressure on the contact surface. This gives a more accurate evaluation of the tire taking not only the rotating mass into consideration but also the lay of the tread. If that fails then go with a balance of the driveline with some new quality joints in place.

Massey

 

It gets progressively worse until I back it down. Gone at 80mph completely and no other speed but 85 to 100+.
brakes no difference. I think the pressure plate may be an issue. When I ran it tail end off the ground the only shimmy/shudder is when the clutch pedal is released.

 

Excellent ideas , thanks. I didnt know about pressure balancing but I thought by getting new tires and moving rims it would change a tire related issue; maybe not. I can get an aluminum driveshaft locally for $100, maybe I should get it. I did new U joints thinking that would be good anyway to no avail but could be a bent driveshaft. I do hear a grinding as I coast to a stop but can't feel anythong or hear anything of it when I hand rotate it in the air. Puzzled on that one too. Thanks for the ideas guys!

 

check out the bushings, I once had a car that did the same and it was just bad bushings in the back. I believe you can pick up a bushing kit for less then 100 or something like that, or it could be from a bad u joint