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.