I put a new trans in my 88 gt t top stang there is a rough viberation
I feel the viberation on the shifter and the floor which causes the panels to rattle
whenever I let off the gas on the highway when I accerlate the viberation goes away even at speeds upto 100
You didn't lose any dowels or bolts from the pressure plate right? They may seem miniscule, but when things are spinning, it becomes more and more obvious.
Is the vibration RPM related? or Speed related? If it is RPM related, I would look into the clutch/pressure plate. If it is speed related, than you need to look at your driveshaft.
Good Luck!
Re-reading, it really sounds like U-joints to me. Been a while, so someone else correct me if I'm wrong, but depending on how you answer the vibration questions above, will really help us to understand the full problem.