Likely corrosion on the piston or could be the emergency brake sticking. The vibration is likely due to rotor being warped from over heat.

Plan of action
Crack lug nuts on wheel in question
Jack entire rear of vehicle from differential
Turn tire to see if it's sticking - With both tires in the air, it should turn fairly easy
take off rear wheel in question
Check caliper pins, see if you can move the caliper from side to side. have someone apply brakes and see if caliper moves on slide pins or if it's frozen
Remove caliper - Inspect brake pads, are they worn considerably more on one side to suggest slide pins or piston sticking
Open bleed screw and put small hose on it into a container. Force piston back into caliper. It should slide fairly easy
Check rotors for runout - Attach something to caliper mounting bracket with vice grips and position tight to rotor, rotate opposite wheel and listen for warped rotor
Remove rotor and inspect emergency parking brake shoes and hardware
Unless the rear brakes have recently been serviced, suggest refreshing them on both sides. If it's sticking caliper, rebuild it, buy new or refurb.
Flush the entire system with something like ATE SL.6

 

Huge thanks, I will get on this momentarily. All four rotors and pads were replaced by the dealer when I purchased the car 3k miles ago. Coated rotors, Etc. Didn’t look like a cheap-out job on their part.

I will follow your suggestions!

Thanks again,
Nick

 

So I spent the majority of the day dealing with this issue. And much of that time was something I was bound to discover eventually...

**** lug nut caps... I was able to get three off without too much damage, but two needed to have their SS cap chiseled and peeled off to get the wheel off the car.

I inspected the pads, rotors, hardware. The pads are equally worn (they literally look brand new), the rotors have no gouges or scoring or any edge lip. No weird colors. No burnt smell on the pads or rotor... Except when removing the rotor and checking the parking brakes...

The parking brake is the culprit! I adjusted the star adjuster all the way in for now and used a rubber mallet to bang them in both directions to recenter them a bit. Cleaned everything with BrakeKleen and compressed air. Relubed the slide pins for the caliper (they were greased fine though).

I drove the car for about 25 minutes and didn’t notice the same drag on the engine as I did yesterday, and no vibration or noise at all.

I got home and did the feel check again. Left rear rotor was cool to the touch, I felt right at the center, where the parking brake shoes ride inside. I checked the right rear and it was much warmer. Not screaming hot but warmer for sure, so something is still causing it to drag a bit, even with the star adjuster backed off all the way.

I have an IIDTool, should I be doing any special electronic procedure for adjusting the parking brake, or inspecting the motor and cabling now?

 

I'm assuming this is your 2012 SC? Lots of info on the Emergency Parking Brake (EPB) issues

 

Yes this is on the ‘12 SC.

I haven’t done much reading on the EPB system yet. I made sure the auto brake was coded off in IID and that was about it.

I have only used the EPB twice since I’ve bought the car, the most recent time last Sunday.

IID only seems to list an Un-jam procedure in the service menu, but I’ve heard there is a service "Mounting Position" as well. I will have to read up more on the adjustment. The rotor was not difficult to remove from the hub at all, and while the rotor was off I actuated the EPB. Not sure if this was a smart move or not, but I saw the shoes apply and retract, and the rotor went back on fine, no clearance issues.

Edit: So I've read the workshop manual. Actuating the EPB with the rotor off was a bad idea, and can cause the actuator to seize up. It did not in my case, so I consider myself lucky. Apparently, I need to adjust the star wheel adjuster until I can't turn the rotor by hand with the rear jacked up, and then back off 8 clicks. Also, bedding them in may be a good idea, and I have read about how to enter the Bedding-In mode.

My issue is that even with the star wheel adjuster completely backed off, I am still getting heat from that side's parking brake.

 

Drove the car again today a bit, a couple hours and noticed the vibration and noise and sluggish feeling came back, but intermittently... It would go away for 30 minutes. I’d park the car, get back in, hit the highway, and alas it would be back.

I just got home and noticed a lot of new brake dust on the wheel and some new markings on the face of the rotor. Maybe it’s the caliper sticking after all? I think I’m going to order a caliper and brake hose to rule it out.

 

So I ordered a rebuilt caliper, set of pads, new brake hose, and while I was at it, a couple new pistons and caliper rebuild kits. I may clean, wire wheel, paint and rebuild the old caliper and keep it as a spare instead of sending back the core.

Also have a new set of 20 OE lug nuts on their way

 

Any resolve on this?

 

I replaced the drivers side rear caliper a few weeks ago for these same issues. It solved the problems.

 

Sorry, I've been so busy I forgot about following up here!

I replaced the Right Rear Caliper with the rebuilt unit, replaced the brake hose as well, replaced the 5 lug nuts, and bled that wheel out. The brake pads were fine, so I resurfaced them lightly with some course sandpaper to get the glaze off the surface. No more dragging.

And as a note, the Left Rear caliper seized up in the exact same way only about a month later... That time I removed the caliper, and rebuilt it using the new seals and pistons I purchased from RockAuto. New hose there as well, as well as 5 new lug nuts, and bled. Resurfaced the pads in the same way. All is good now.

If one is on it's way out, its likely the other will probably not be too far behind.

Hope this helps!

Nick