used to be a rubber coil spring bushing the dealers would use on the driver side to correct the lean.

some times the frame is bent from flexing when the crossmember lower frame brackets are removed when R&R of the trans. they seem to never get put back on most trucks.

the lower edge of the frame will be bowed in the brace area.

the frame flexes a lot at the steering box and the factory used a box/frame brace there to stiffen up the frame.

if frame is bent and crossmember brackets MIA it needs to be on a frame rack to get it back to specs.

another issue is worn rubber cab mount bushings,I guess you could add washers/shims to the body mounts.

I know my poly mounts leveled my cab when I replaced it on my 85 truck.