97 Suburu Outback locking up

97 Suburu Outback... Locks up driving forward as if brakes are applied in about 20 ft. in reverse, no issues whatsoever. Mechanic has the rear diff out and looks completely clean with no evidence of heat or abnormal wear, rear brakes the same. All bearings on the rear axles are in great shape. The only oddity was that the diff vent was missing... looked to be "blown out" indicating it was plugged (possible cause of locking up?). Prior to going to the shop, it did not do this consistantly... my wife drove 50 miles to pick me up at the airport and leaving the ariport it completely locked up and had to be towed. several days later I drove it to my mechanic (slowly!) and had no issues. any ideas????!!!!! Trans???  BTW, tires are all the same size and wear, so it is not a mismatch of tire size issue...