First you need to find out if its coming from the diff, or maybe from the brakes. Fluid level check in the rear diff and looking for any metal flakes in the fluid would also be my first guess.
If you have quadra drive, it also could be lack of limited slip additive in the diff. Was the fluid changed lately
But to help better, we need to know if it happens always, or only at certain speed(s), accelerating and-or decelerating etc.
But also take care, that certain sounds, which seems to come from the rear, also could come from the T-case for example. The sound can be transferred by the drivetrain to the rear.
JEEP Grand Cherokee 4.7 Lmtd 2004
HID 8000K in low and fog / Quad Squad member
KYB Gas-a-Just shocks / Pirelli Scorpion ATR 245/65/17 - Transgo reprogramming shiftkit.