I had a similar issue. It sounds like you have something heavy sliding around in your trunk when you upshift.

I'm pretty sure it's bushings.

Is the butt end of your car sagging as though your springs are tired?? If this is also true then my guess is it's the largest bushings that are actually pressed in to the axle. there are two arms that mount to the top of the axle and run toward the front of the car. they too get tired but my clunking was those big babies that the arm attaches to (on the bottom of the axle). I got corageous and did them myself. I did the bushings in the rod that runs across the back of the car, that didn't help. I did the two rods on top of the axle (that I mentioned) and that didn't help. The big ones, I think that's your answer.

If you don't have a way to press them out, take it somewhere. I used my friends service station and it was tough to say the least.

Your car has a few things that can cause this. First there is a rubber

dampener instead of a front u-joint. This is right behind the trans.

The next thing is the rubber driveshaft support and the bearing inside of

it. These go bad and the bearing can seize making vibration in the drive-

shaft. Of course you may have a bad u-joint. Another possibility is the

torque arm bushings in the rear suspension are bad and letting the axle

rotate causing the driveshaft angle to change also causing vibration. I

would do a careful inspection under your car. If any of the rubber parts

are bad you can usually see that.

If the sound/vibration changes depending on the gear you're in, suspect the trans. Conversely, if the sound/vibration is fairly constant regardless of the gear, and does it under any load (accelerating/decelerating), suspect the rear end.

The M47 is fairly solid; the read axle, however, eats up drive pinion bearings.