Driveline noise

I've noticed a noise, that sounds like it could be a U-joint...it squeaks at low speeds. I have tried moving the front a rear shaft and it seems like all the U-joints on the jeep are fine.

But lately when I down shift(AX15)from say 3rd to 2nd(using engine breaking)...I hear a loud sqeaking as I'm letting the clutch out...and it seems to continue as the engine slows the jeep. I also notice it when accelerating also but not as much due to exhaust(flowmaster 40) and TSL's. The jeep also makes the noise when I am reversing. The noise goes away above like 20mph.

I noticed some melted platic on the back of the t-case where the drive shaft goes in but I'm pretty sure it has been there for a while.

Any clue as to what could be causing this noise? I'm hoping its just mud in a seal somewhere, that will just go away.

mud in a seal will destroy the seal, not just go away. Anyway,
try pulling your rear driveshaft and driving in 4wd.

Just letting everyone know...it ended up being the u-joint in the rear(had no bearings). A ball joint press made the u-joint swap pretty easy.

The driveshaft had no play side to side which is why i thought it wasn't a u-joint.