right rear thud

I've got a dull thud or thump sound coming fom the right rear end whenever I go over a sharp bump, like a speed bump. It's especially noticeable if I'm backing up over the bump, like backing into my barn. It sounds exactly like there's something loose in the trunk, so I made sure the battery and spare tire were tight, and the sound is still there. I've read about the rear differential clunk, but I don't think it's that because it's related to bumps, not whether the tranny is shifting or not. Any ideas what to look for?

Get a close look at your rear springs. They break pretty regularly.

If it's not the spring, as jonathan mentioned, pull out the grey covering in trunk on the right side, and look at the upper shock mount. If it moves when you bounce on the bumper, replace it.

Thanks for the ideas, guys. I'll check it out this weekend.