I've had a thunk/pop noise for a while now on hard braking and when reversing up an incline (like backing up a driveway).

Thought it was something loose in the suspension, but word from the shop just came in and it turns out it's the driveshaft where it meets the transmission output shaft. It slips back and forth just a little bit.

Now I've got an LPE aluminum driveshaft, but this didn't start right after that, I ran the car with the LPE shaft for a good long while without problems. It did start not too long after a transmission rebuild.

Any thoughts on how to cure this? I figure that this could be caused by the driveshaft being just a smidge too short, but it would seem that if that was the case it would have turned up right after the driveshaft install.

Next theory is that the engine and or tranny mounts are bad, letting things shift forward just a little bit, just enough to have this play in the driveline. The engine and tranny mounts are oringial 1999 128K mile parts. But the shop says they look fine. And they couldn't find anything loose in the suspension either.

Could it be caused by the transmission rebuild? (that's why I posted this in Transmission)

Thoughts?