I would recommend Gore shorts. I have a similar problem in the front, and Gore chamois fit the bill there. I think it's the chamois shape and the fabrication that really reduce chaffing for me. Unfortunately, the chamois is too big in the back for me and the shorts themselves are all too long on my short legs, so they might be perfect for you. They aren't usually cheap, but they are really nice quality.

In fact, what size are you? I may have a pair that are basically new that I could give you, if you'd like.