This is the left side, pick taken from the trunk. This side is not the issue.

This is the right side, also taken from the trunk. This side is the problem side. When I hit bumps, or the like, I get a solid banging sound.

I have scoured two different shop manuals and fora galore. I can't seem to find anyone else with this issue. Can you lovely folks help me diagnose and suggest a fix? I think it may just require a re-weld of the body in that area, but I wanted to ask the experts first!
Thanks!