My car went in for a major service this-smorning, same prob + a few others.
Basically the rear drum pads are not adjusted properly, the pistons are getting old and the drums need machining. They were badly adjusted by previous person who did them *grr* so they were being pushed into the drum incorectly,causing incorect wearing and everyting that goes along with it.
To fix ->
2x new drum break pads = $40 total
2x new drum cylinder thingies (push breaks out) = $30 total
2x machinig of both rear drums = $ 40 each