Theres a good chance the cab mounts are deteriorated. There are 4 or 6 bolts that hold the cab to the frame, and they pass thru rubber bushings about 4 inches thick. Those deteriorate over time, and may have rotted away completely in one spot causing the cab to sag.
Look under the hood at the suspension set up and see if the caby sheet metal is closer to the frame on one side than the other. You may be able to slide underneath and see if the cab body bolts are loose or broken also.
Also, some of those ealier trucks were sprung with front leaf springs.. One of those may be weaker than the other side also.
ETA; See if you can shake the cab without shaking the whole truck (doubtful because of the other mounts, but sometimes you can) Also, you can slide a jack under the cab and see if you can jack it up and inch or two without jacking the truck up off it's suspension. Most off-road rebuilds of these replace the rubber mounts with polyurethane bushings that stand up much better.