[rotorwrench]

When troubleshooting a problem, I like to start with the easiest thing first. I'd take the trans shift tower off and have a look in there first. Move gears through to check each one for problems. If it all looks OK, that's the time to support the car, disconnect the whole rear drive system from the frame & components, then pull it back to inspect the U-joint. There is only one bolt that holds the U-joint to the main shaft coming out of the transmission. It should have a lock washer along with the large washer on that bolt. If properly torqued, they seldom come loose but never say never. Stuff like this can happen. I would wonder what lube was used for the U-joint. Some lubes work and some don't work well.

As was mentioned, there are the OEM bushing type U-joints and there are Lincoln type needle bearing joints. I'd stay with the bushing type if at all possible. They are more reliable, believe it or not.