I've been chasing a super annoying squeak in the back of the cab for about a month now. Have had very few chances for someone to drive so I can troubleshoot.
It was a long squeak over the tiniest bump. Even stopping at a light, it would make noise from the body rock.
I started by pulling the rear seat and surrounding trim, tightened all the seat belt bolts, and rear window nuts. Nothing
Opened the rear pass-thru window and heard the tailgate squeaking. Score! Purchased Needa 38424 bushings from Advanced for $4. Fixed the tailgate, but the squeak persisted!
I finally got annoyed and started pushing on everything I could.
It was the lower weather trim surrounding the glass. I took a shot and assumed that there was some dirt stuck between it and the body, especially since all the glass bolts were tight.
So... the fix? I found a Velcro cable tie behind my desk that I wasn't using and ran it back and forth behind the trim. There was a surprising amount of crap back there.
No more squeak! My life is now complete. Hopefully this quick and cheap fix will help others from pulling their hair out.

 

I think I have that squeaking.. can't seem to find it, maybe I'll give this a shot, thanks!

 

Hmmm, you have reached rock start status! Tough, tough find.