Rear Tires Are Wearing Unevenly......

X2. Find the camber and toe for the rear wheels. Do you have the printouts from the previous alignment sessions? May save you a trip.

You can also try to measure camber yourself with an angle gauge like this one or with even a smartphone app. Inflate the tires to the correct pressure and park on a level surface. It won't be as accurate as an alignment jig, but it can catch the problem if the camber is grossly off.