I recently noticed a very faint "whump whump" from the rear of my 2005 when I would let off the gas at slow speeds. (It is so faint that maybe I just don't hear it under power or at high speeds.) Having no idea what it was I jacked up the rear end and started spinning the rear wheels. When I start spinning the back tires and then stop or reverse direction I feel some freeplay. When I spin the wheels, the rear wheels move freely for a .25 centimeter before the axle moves with them and there is a bit of a "clunk" sound as soon as the axle gets engaged to move.

Is this a problem, and if so, how big of a problem is it? I'll be selling the car in two years and I only put about 3,000 miles a year on it. They are almost all tough, off-road miles though.

Sounds like an axle bearing or spider gears. Then05s did have carrier issues .