So I recently had the clutch replaced in my '91 240. Not long since then, during a spirited shift, I'll hear a pop that sounds like it is coming from either the u-joint or the differential. The thing is that the driveshaft was not removed during the procedure. The guy just disconnected the driveshaft from the transmission then dropped the tranny to get to the clutch. What is going on back there?

 

Get under there while the car is securely on jackstands, in park, and grab the shaft with two hands and try to get it to rotate. If there is any play at all, inspect your u joints, and if they are at all worn replace em. Make sure everything is snug, wouldn't hurt to try and tighten u joint bolts while down there, but don't go crazy over tightening them.

Or maybe you're just not very good at quick shifts...

Good luck and let us know.

 

I'd look at suspension bushes, particularly the torque link bushes next to the differential housing.