2003 Subaru Forester No power to rear wheels, low whirring

Drive Train Axles Bearings problem
2003 Subaru Forester 4 cyl All Wheel Drive Automatic

Noticing a low whirring noise from the rear, our mechanic ran it on the lift, and noticed the rear wheels not turning with the fronts, they just freewheel. The tranny got hot. He flushed it and changed the fluid and filter, gave my wife a bill for a buck sixty-six, and said he was unsure of why it might be that way. I'm looking for what the likely failure may be and the most sensible way to remedy it. The car is otherwise in excellent shape and has about 100, 00 miles.