2004 Sunfire rear brakes weird one!

Mileage: 75000 km bought this car seemed good till I hit the highway and applied the brakes, massive vibrations from the back brakes. Had the rear drums and shoes replaced. Vibrations gone but now the right rear brake squeeeels! Took it back, had them machined, good fo a couple days then same same. Have had the rear brakes done now four times (no charge) but still they squeek. Also the right rear wheel locks up b4 any others. If I reset the brakes the noise goes and the brakes dont lock up but the probs always come back. My mech is at a loss as he cannot find anything wrong after repeated trys. Right now it is just a noise issue. But with winter coming I cannot have that rear wheel lock up like that. Any ideas?