[joyridin']

What type of rear brake set-up does it have? It could be a few things. Bad caliper comes to mind first regardless whether it is new or old. Bad hose might also be another option. I doubt it would be anything like a bad master due to the fact the other wheel is free. There generally is less play in the rears than the front, but not much.

Are you sure both pads are hitting the rotor? My other thought might be the caliper is frozen on the mounting.