Was a solution to this problem ever resolved?... I have a very similar problem. The other day I was driving home at night and had to go hard on the brakes as well to avoid an accident.... the pedal became hard to push after some time and the car vibrated and was sluggish.... I was able to get it home but knew that one or both of my rear brakes were not releasing properly and had progressively gotten worst as I got off the highway and into city stop and go driving. Luckily, I don't live far from the off-ramp but by the time I got to my driveway, I could hardly press the brake and the car had a hard time rolling. The next morning, I jacked up the car and both wheels rotated without and issue... it seemed that the "line lock" released but not sure as to how or why that happened. I'm afraid to drive it now, but not sure where to look first.
Any insight to this issue would be greatly appreciated. Thanks! -Jay