i have an '81 corvette with the exact same problem. i have changed all 6 u-joints twice, balanced tires, etc etc etc...to no avail. the engine is stock as well as the trans.
at 55-65 the vibration comes on, especially upon accelleration, throw it in neutral...it coasts smooth as silk. bring the engine up to same RPM's while in neutral...no vibration.
i'm assuming it has to be in the transmission....but before i bring it to a shop, i was hoping to at least know what to look for.
if it is the transmission, what could have gone wrong?
