I agree with what Michael says. To test whether it's at the calipers or in the front most part of the cables by disconnecting the parking brake cable at the T under the driver's door. With that disconnected you should be able to see whether the problem is in the front most cable or not. From there I'd pull the rear wheels off and check the slider pins on the bracket. If those are not frozen I'd be willing to bet that one of your lines is crapped inside.
Also take a screwdriver and work the release on the calipers and see how easily those are moved. There should be a good bit of pressure on those to release from the banjo spring. If those are slow to move I'd lean toward the rubber brake line(s) being bad.
You might also want to PB Blast where the cables go through the caliper housing. Those often times will get stuck in there and make it very difficult to remove them if you have to remove or replace the caliper for some reason.