Creaking noise from rear end?

Warmer weather has me now rolling along with the windows down and the rear slider open. Noticed that there is a "creaking" noise coming from the rear end of the truck that you cannot hear with the window closed. I do not believe it's the clunk, as I've already had that particular issue with the original drive shaft and it's been replaced under warranty. To the best of my "hearing" I do not believe it's the factory leafs, as I've religiously greased them using Bird's method. I can get on the rear bumper and bounce the truck up and down a little, and no noise. This only seems to happen when reversing or as I'm pulling away from a stop. It does not seem to persist in 2nd through 6th, but then again, lot's more wind noise to help drown it out. I'm quickly approaching the 60K mark and if it's drivetrain related, would obviously like to get it covered while I can. Just looking for feedback from the group at this point so I have somewhere to begin looking on my own before taking it to the dealer. Axle wrap?

Thanks in advance.

Master Yoda

What condition are your rear shocks in, any leaking or have you had them rebuilt yet? One other thing might be a loose frame harmonic balancer. It's located on top of the drivers side rear frame. Right behind the rear wheel and underneath the bed. Good luck.