This problem started a few day ago. Backing out of a parking spot and all of a sudden I hear a clunk from the back end as if I hit something. It feels as if something is binding up then releasing and it only happens in reverse. My thinking is that it may have something to do with the rear brakes because I've also been getting a groaning noise at low speeds when engaging the brakes, but that doesn't happen all the time.

I changed the rear pads a couple months ago and did have some trouble getting the driver side piston spun back in, but at the time I thought it was because the caliper tool kept slipping from the slots in the piston (More my fault than tool). I'm wondering if that may have been a sign of a bad caliper or something.

Any ideas would be helpful as I'm not so adept at figuring what the problem is, but very willing to do the work if pointed in the direction of what to look for. Thanks.

Guess your going to have to go back and pull both wheels off to find the culprit and figure out where/what is grinding.

Mechanically, I can understand the grinding noise with drum brakes, but with disc brakes, I can't comprehend it.

Is your emergency brake working? Might be worth a few moments to check your rear stabilizer links- make sure one end didn't come off due to corrosion.

Parking brake is working. Stabilizer links not broken. I actually have to fix the front passenger for that reason. It started after I hit a pothole really hard, so maybe suspension related. For some reason I didn't put that together sooner. I noticed tonight it feels as if the rear end is trying to climb over something. I can actually feel it raising up. I didn't push it past the first initial resistance. Not good when you're trying to back into a tight spot in DC.

Check the brackets for the stabilizer links bolted to the frame? I had the same symptoms and it ended up being a broken bracket.

you should take the drums off seems like something in there is hanging up

This model year has rotors in the rear (except for the mini drum built into the rotor for the parking brake).

Now that you guys mention that, I had a very similar situation with a friend'* car. It had one of those park brake set ups where it was a brake shoe internal to the rotor. In this case, the park brake shoe had broken free from its anchor point, and was floating free inside the rotor. Driving forward, there was no issue. Backing up, the loose shoe would bind and make it feel like you were backing up over an obstacle.

In short, it was doing exactly what you are describing.

Just put it up on the jack. Lower bolt that connects the caliper braket to hub is gone. Somehow sheared off when I had that hard hit. Is this something I can easily find at a parts store? All I'm finding are the slide pins and caliper bolts.

maybe, might be easier to find at a junk yard or gm dealer