Look at the cab to frame body mounts. My bet is the rubber bushing has pulled through the frame mounting hole, or the rubber has deteriorated enough to pull through the frame hole. I've seen this happen on frames that still look brand new, its not a rust problem as much as a wear problem.

The area around the rubber sits on on the frame bracket is only about a 1/4" wider then the outside of the rubber mount itself. With age, dirt/sand/grit gets between the mount and the frame and wears on the frame's metal surface. When you get a jolt or sudden movement, the edges of the frame bracket give away very easily. To repair the area, you will need to remove the bolt, jack up the cab, pull the rubber bushing out, and replace a section of the metal around the hole. Basically you weld in a ring that is about a 1/2" wide to the existing frame bracket so the rubber has something to sit on again. You may or may not need new rubber bushings. I suspects you will need to repair at least both body mounts on the driver side, but I would also look at the passenger side as it may not be a lot better.

In my shop, that is about a $100 repair per mount, + the rubber bushing if it also needs to be replaced. I have done a lot of them. Its more an issue of the design rather then a manufacturer, everyone uses the same design. Gene