Either the wheel bearings or the 3rd member...( differential )..... check the the fluid there first.... but better to just drain and put new in.... be sure to loosed the top fill plug first... IF that is stuck I have posted low tech but high pressure way to loosen it in the archives using the jacking hole in front of the rear wheel and a comealong.... don't want to use big power on that area ... or heat... slow application of power up to the necessary point is the safe way.
At the wheels bearings.... you might could jack up both rear wheels and turn them while a stethescope is placed near the bearings ... or you could just preemptively replace them... and sleep knowing they are in good shape for a long time.