Check to make sure your rotor isn't bent, and your pads are installed correctly. Also, you may want to check your rear master cylinder. If the cartridge in it is bad, that is often times the effect it will produce. If your master cylinder is good, you may have a bad caliper piston. I have never really heard of them going bad, but if a piston bore was out of round or scratched, it could cause the pad to drag on the rotor.
Well, the pistons should be somewhat tough to move with just your hand, however, it should not be that hard to push a small rear brake piston back in. You may want to pull the caliper back off, inspect the piston AND the seal that goes around the bore. Perhaps the seal is old or twisted, or you may have even gotten a small piece of debris in the bore. If you remove the caliper, the pistons are easy to remove by shooting some compressed air into the inlet where your banjo bolt runs through. Make sure you use a piece of cardboard or something to catch those pistons, as a good amount of air pressure will actually shoot them out of the bores.