Can the cab bushings on a 1988 ford f150 squeak if they go out or can they even go out?

Yes, body mounts can rot out and can also cause squeaks and creaks while driving. It could also be your springs, front or rear. Your rear leaf springs have a few areas that can make noise. At either mount block where the eye of your spring is bolted to the frame, or the leaves themselves sliding on each other. Older vehicles had grease fittings on the eyes and sometimes even on the leaves. I would start there, spray some WD40 all over the leaves and get it into the eyes of the springs too. Then, well...see what happens. All you need to do to diagnose the body mounts is look at them...if they are cracked and falling apart, they are finished.