I did some towing with my jeep. I noticed the back sagged a little after the years I drove it. Actually with the trucks I've had I've noticed that too. I think those leaf springs just let the butt sag after years of use. One side sitting lower (left to right) is odd though... First place to look is a broken spring, I'd say.... and xntrik is right, it could be a coil in the front as well...
How does the car handle? Does it pull, is it loose on the highway? What does the tire wear look like?