Rears are hard to diagnose. First thing I'd do is get the car up on jackstands and let it spin while listening. It just may be an outer axle bearing, which will require pulling the cover to release the "C" clip holding the axle in. Gears usually make noise either accelerating or coasting and there are different reasons for each. Looseness at the companion flange is a dead giveaway as is oil leakage. You HAVE checked the oil, yes?
