i know everyone is gona dissagree but this happened a few weeks ago to me. i replaced the shocks, witch were shot, and it fixed it. the right rear shock didn't expand by itself at all and the left was in a little bit better shape. i put a set of new bilstein's and the rear leveled out.
I have skyjacker leafsprings and belltech shackles and new fox shocks(no blocks) and my trucks right rear was an inch lower then the left rear
so i used different holes on the shackel for each side of the truck, and now sits perfect