You know what's awesome, I haven't really messed around in the bed for a while, but I was getting a new tonneau put on so I took the old one off. Damn squeak starts driving me nuts. I track it to the top bed rail on the tailgate you know the "bed liner". It slides a little, get the silicone spray, psssst, gone. then I push on the panel, squeak, pssssst, gone. Then I close the tailgate, creeeeak, whip out a second can of liquid wrench and go at it 2 fisted with people looking at me like WTF? pssssssst, psssssssssssssst, pssssst, pssssssssssssssssst, gone.
I haven't really maintained the trac much in the past 3 years, and man I'm going front to back with maintenance now before it rolls 100K. I missed all the fun futzing with everything on this truck, but now I think I'm going overboard.
That loud creak is 99% the rear bushings or links. If it was a chirp, your tire needs to be cranked up. Clunk, check your back seat floor bolts. Clack, something in the bed. Whine from passenger seat, wife.