Did you but the truck new or used?
I bought a 1/2 ton truck used and the left side had a lean to it. If you would walk in the bed and bounce the left side was "softer" than the right. After looking at the truck I think they previous owner hauled with it and one of the springs went got soft. I ended up upgrading both the rear springr to 3/4 springs and it cured the problem! It also made the truck handle ALOT better. The truck always had real bad body roll. Id check the springs that could cause the whole side to lean. Try what I said and walk in the bed. You can look for cracks in the spring but the spring could be bad and not have a crack. If you do think the spring is bad Id replace them both at the same time as well.
