I assumed that it was caused by 15 year old leaf springs that were just getting tired, so I put a set of helper springs and new shocks on it. Rides better now and did level it out a bit, but still have a noticeable amount of drivers side lean to it. I happened to notice that one of the spacers (drivers side) between the frame and the bed is crushed. My question is: Are those spacers replaceable or are they a permanent part of the bed? My only other option here would be to put shims in and try to get it as close to level as possible, which my father-in-law did up front before I got the truck. lol

 

Here are pics of the brackets closest to the tailgate. Passenger and Driver side.

 

I have the same problem, but it's my cab that's off kilter. When I looked under my truck I could see the old body bushings were either rotten or missing altogether. So I think I'll replace them with hockey pucks and call it good.

 

Easiest solution would be to loosen the Bed bolts closest to the problem area and and weld in shims.
What I would do is remove the bed and cut the weak rusted spots off and weld in heavier pieces and well I was at it clean the frame up a bit and under the box and paint it with some rock quard or bed liner.