It HAS to be the suspension...somewhere. Check those drop shackle angles on both sides. Are they the same. Measure from the center of the wheel to the flare instead of the ground. The surface is relative, distance from the wheel is more telling.
I'd check for a missing or broken leaf...or maybe a missing or collapsed coil isolator. Then start looking at your "uni-frame". Look between the cab and bed for a bend or break in the frame rail. Look at your front leaf mounts too, bushings??
My 88 has a 1/2 lean on the driver's side as well. I am pretty sure it is the coil spring isolator because it looks pretty well compressed compared to the passenger side. How hard is the isolator to remove?