My 85 300SD with nearly 300K miles has developed an unusual problem. When I accelerate from a stop or up an incline it will sometimes make a brief sound (slipping/skidding)from the rear wheelslike it is losing traction. It feels like it's losing traction as the car seems to hesitate ever so slightly.It lasts just a second and thenI continue on.My mechanic can reproduce the problem but has no idea what causes it. Rear tires are good. Rear shocks seem to be good. My driveway is relatively steep and the problem will often occur midway up. After it happens I discovered stepping sharplyon the throttle can cause it to occur again.Any ideas would be welcome. Thanks.