Ok have a 2000 F150 reg cab short bed, 4x4, auto, 5.4, has 109,000 miles on it and I just started to get a squeak from what I'm pretty sure is pass rear. Before I just start replacing parts just wanted to see what you guys might think it is.
I can easily hear it in the cab everytime I hit the slightest bump, its never gets louder or softer with speed, it does not do it when I hit the brakes, I jumped up and down on the bed and heard it slightly, only thing replaced in the back were my shocks with ranchos about 2000 miles ago.

 

Maybe the Leaf spring bushings? There is like pads you can put between the springs to keep the metal on metal from rubbing .