My back right wheel locks up while backing up, any suggestions on what the problem is or a possible fix?

I have a 1993 Cadillac DeVille. All the wheels seem to work fine while in drive, but while in reverse the back right wheel locks up. Two things have happened recently that could be the source of the problem. First, we just had a flood in the parking lot two days ago, so maybe rust? Second, we just had that tire replaced and the rim fixed (the rim was bent up and Rent N Roll bent it back with a lead hammer). I'm hoping there's a cheap way to fix this but I'm pretty sure I'm going to end up having to take it to the shop. I called a shop and they said it might free up with a tow. I called Rent N Roll and they said they don't see how what they did could have anything to do with it.