So did a thread search on this already and have a few areas to check (break lines, springs and muffler shielding) but though I'd still post to see if anyone else has any ideas. I had a 2.5" RK Flex installed about 10k ago with Bilstein 5100s. Running 35's with 4.5" backspace. Squeak is very high pitch coming from rear passenger side while driving (accelarate/decelarate makes no difference; braking makes no difference). Occurs over small bumps and imperfections in road vice large bumps. I haven't done any real crawling since the install due to work.

I crawled under and was able to press up on the frame and trailer hitch to recreate the sound. I don't think it's the shock but definitely up high near the tub somewhere. Any ideas would be appreciated, driving me up the f'ing wall! Still searching....

 

I have EXACTLY the same symptoms!! Does the squeak go away when the under side of your Jeep gets wet? Mine does, but then comes back after it drys out. I can also recreate the sound by torquing on the lug nuts of the rear right tire with the lug wrench. Don't even have to torque hard, so I thought it might be an issue somewhere along the axel. Still haven't found the culprit though...

 

Hasn't rained since it started. I sprayed silicone lube on rear control arms, track bar and sway bar joint. Then checked break line clearance on rear passenger side, which was rubbing, so I bent it slightly to prevent rub. Squeaking stopped...at least for now. I think it was one of the control arm joints but could never really pinpoint where it was coming from.