U Joint

things to check:

1..what kind of shape is the trans mount. at the bareminimum i'd get a poly one.

2..What is your pinion angle set at?

3..what size of joint are you using (the small a body one?)

there's three things that could be doing it

Also do you know there is a correct way and incorrect way to install a u-joint. The grease fitting must be in compression not expansion when the car is moving.

don