Mine was also sitting crooked. We discovered a bend in the rear passenger side frame rail right at the top of the arch leading down to the back of the tail panel. And this was after the body work was done and getting ready to be painted.

I took it to a guy who straightens frames and he completely pulled it out and it sits perfectly even now. It did make a crease in the top of the quarter panel ever so slightly but we just had to mud it in a little to make that work.

I was thrilled because I can't stand being behind a car on the road and seing a crooked rear end.

Even worse is a truck that has the bed cockeyed with the front cab. It looks like a huge sandcrab going down the road.