OK, let me see if I understand this: Leaning to the driver side is obvious, but what about "it leans to the front to the back" ?
Does this mean the left front corner is low, the left rear corner, or is the truck "swaybacked"? (Frame bent, sagging in the middle)
Regardless, I would check for the following: Collapsing cab/rad saddle mounting points due to rust, tired or missing cab cushions (the rubber "doughnuts" at the mounting points, there are 2 on the rad saddle and 4 under the cab, tired or broken spring leaves, and on the pickup box collapsing crossers under the floor, again due to rust because they are just formed sheet metal, same as the cab/rad saddle mounting points.
It may well be a combination of more than one problem making the "lean" visible.