First off, check the bolts connecting the tailshaft to the diff are tight, mine weren't tight enough once & i got similar problems. If this isn't the problem, & you're already under the car, pull the tailshaft out & check the universal joints. They should move freely, no binding or tightness. If they're worn, replace them, if not, then start looking for any thing else that you might not have tighten enough, just do a spanner check over all the bolts. Once you know everything is tight & the uni's not worn, then check that the engine isn't crooked in the engine bay, or that it's leaning too far back or forward. the engine & transmission is supposed to be square in the car,even if it's offset one way or the other, & even with the ground. You might have to modify some of the brackets to get the motor correct.