on my 96 f150 long bed 4X2 there is a squaking noise that is made between the bed and the cab. it apears that it is coming from this peice of metal that comes of the rear of the cab that is bolted on to it. and that bed is rubing up against this bolts when ever the truck goes over any oneven surface.

 

The plate is the weight that keeps the back wall of the cab from drumming. Sounds like either the truck has been wrecked, or the body mounts are going bad, or the bed bolts are rusting away, or the bed front is dented forward against the cab. Only you can find out for sure...