Are you sure its not your butt cheaks rubbing together?
(sorry, I can't believe I just said that, but what the hell? It's late..)
It could be tons & tons of different things..... You need to see if you can reproduce it.
#1) Bounce up & down on the suspension. Can you hear it?
#2) Take the truck to a carwash and spray out the rear leaf springs good.
NOTE: When the leaf springs are wet, the squeek WILL stop. Wait a couple days until everything dries out before you think the problem is gone.
I've got squeeks too now. But its the rear shocks squeeking when I'm running heavy loads.