Sounds like you have a frozen piston in the rear brake or a broken return spring. The issue from my perspective is that any frozen disk pad generates alot of heat along with the noise.

That said, the heat can tend to overheat the rotor, causing it to warp, it can overheat the brake fluid in that caliper's line and make it near worthless, the heat can destroy the caliper piston seal(s) and fry the pad lubricant.

So, in the case of a hung up piston or pad:

A) mic the rotor and check for warpage and excessive wear, turn if possible or replace the rotor if necessary (worn beyond turning specs).
B) flush the brake fluid in that line and perhaps the entire brake system (you are going on 3 years now anyway so it is due).
C) replace both pads.
D) rebuild or replace the rear caliper based upon its condition.
E) check the bearings if the heat was excessive, it can fry the, and the grease, check spindle as well.

If you can get all of that done for $90 consider it a gift... that is alot of labor.
Recently a guy at work had this same thing happen with his Mercedes, the bill was $590, he needed a new caliper, bearings, rotor and pads and fluid flush. He had to wait 2 weeks for the rear caliper.

Be careful amigo... once they see the ROTEX pads your dealer could raise more than an eyebrow... BTW, who installed the ROTEX pads???