Rear brake problems

Two weeks ago I took my 2004 Prius (with about 165,000 miles on it) to the dealership because of problems with the rear (drum) brakes grabbing (sometimes hard enough to lockup one of the wheels). Their diagnosis was that they were badly out of adjustment. They "cleaned, adjusted and beveled rear brakes" which fixed the problem, but it returned this morning. I should also mention this dealership had "cleaned and adjusted" the rear brakes twice before over the last couple of years.

Does anyone have any ideas what could be causing this? My guess is that it is related to being illegally towed (pull onto a flatbed) a couple of year ago. Since the emergency brake was set when it was towed,
could something have been bent that is causing the rear brakes to go out of alignment?

I took my Prius to the dealership this afternoon to replace the combination meter (it started acting up yesterday afternoon) and to take another look at the rear brakes. I don't expect to hear back from them until tomorrow.

If any of you Prius experts have any ideas about the brake problems, I would be very interested in hearing them.

Michael