Since you couldn't depress the pistons on the rears(and I see you said turn, so it's not that you're trying to just push them in like the fronts), I doubt it's the prop. valve or anything other than the calipers themselves. I'd get some rebuilt ones. Might find a good deal online or hit up a local parts place.

If it was just that the pads were potentially sticking, I'd lean towards taking a look at the e-brake mechanism on the calipers (or maybe even the MC/prop. valve) but since it's so uneven and, as said, you couldn't get the pistons to move, I would lean towards the calipers. Not a mechanic but that's my guess.

When was the last time the fluid was bled on the car?