Before you blame the frame, check everything else. I have found that even under normal use, identical springs sag differently. Plus bushings can stick and bind, the sway bar can get tweaked, etc. Plus, a problem in any spring can throw all 4 off so it's hard to tell if the lean comes from the front or back. You can use a cut overload spring for 3/4" or a 1" add-a-leaf to tweak things. The only problem with this is that if the original problem is spring sag, you are getting a bit of "steer" in that axle. This is a bigger problem up front where the "saggier" spring will tag the fender sooner even though the truck sits level.
I find myself tweaking something every year or two to keep the rig level. I like to measure from the axle tubes to the frame to check the springs and then from the ground to the bodylines. It's important to take the measurements parked in a few different locations unless you have access to a surface that you know to be really flat.
Sometimes a single hard manuever off-road can change one of your springs. You could always run coilovers and adjust them all at will.
