I have exactly the same problem... I will change my u joint and I bet that is the problem.. The little bearings get out of position if not installed correctly..

The other thing would be that the differential is not balanced or something and i dont think that is the problem, unless you have chipped teeth or something.

 

i'm having vibration problems at 65+ as well...and guess what, it's a 94 z...

here's the thing though...it worked fine before i changed my transmission...i could take it to 150, and it would be as smooth as buttah!!

now, it's not like that...so, something changed after that new tranny was installed...

 

Do you have a T56 tranny? why was it replaced or rebuilt? In otherwords....EXACTLY what changed in your car? We may be zero'ing in on this problem.

 

I had similar vibration problems when I bought my Z. I actualy had two separate problems. The first was a high speed vibration at 85mph and above. That was the driveshaft vibration common in our cars which I solved by installing a GMPP aluminum driveshaft. I also put in a polly trans mount after breaking two stock ones. The polly mount added some more road feel or harshness but not realy vibration you could feel in the wheel.
I still had a low speed vibration around 45mph. What was causing that was the engine. I had a leak in the intake elbow and once I fixed it, the vibration went away. So I can tell you from experience that not all vibrations come from the drivetrain, they can come from the engine. My car is smooth as silk now which at times I thought was impossible in these cars.