My guy has the same problem.
My first dog had it, too, and as he aged he developed a wicked swayback.

Eventually, (at around age 14), his hind legs became weak and he would fall. Didn't stop him, though, but he was a hard one to get near water---and I agree that swimming is probably one of the best, low impact exercises you could do.
I know very little about genetics, but I think it's a genetic defect, so I'm actually not sure how much you can change, other than to strenghten the area to try to prevent injury or deterioration.
Look forward to hearing ideas/suggestions!
