Drivetrain vibration fixes? U joint? Grease?

[98_Q45]

So now that everything seems to be running flawlessly with the Q, there’s something a friend passenger of mine noticed last month...that’s now got me on alert and noticing as well.

Whenever I accelerate and hit 25-30, there’s some subtle vibration coming from the drive train. It’s not a sense of tire out of balance. It feels like an undercarriage shudder, but I’ve checked and changed the transmission fluid, and all seems to be well. Once I get past the 1st gear, it’ll smooth out. Highway driving seems to be intermittently smooth...it’ll seem smooth until I hit a bump or rough road, then kinda feel rumbly for awhile until I hit smooth road or continue driving.

I’ve looked up something about “u joints” in RWDs, but I’ve inspected my boots and nothing seems torn or leaking in the back. I’ve also changed my differential about 3 times in the last 75K or so miles. Initially it seemed like it was neglected, but subsequent changes (regular Vavoline 80w90 with a switch to 85w140 in summer) have yielded relatively clear fluid.

Just wonder what I might be looking at down the line...replacing whole axle shaft or just joints.

[BCC93QT]

Exact same issue I had with one of my G50 Q45. The driveshaft on the 1990-1996 has a semi common failure of the bearing that connected the 2 shafts. Not sure on the FGY33. The 94-96 you could replace the bearing versus the 1990-93 you could not. Put the car one the lift and inspect. If the boot is torn and worn chances are it needs replacement. But the bearing may still be bad with a good boot...