At first both rear brakes weren't contacting the rotors equally. There was just a nice clean line in one part of the rotor where the pads were hitting. I thought the rotor was warped so I replaced the rotor and pads. Then the caliper started sticking and heating up the rotor - the pads would just glaze over.

I then replaced the rotor/pads caliper, and caliper bracket, the chrome pad plates and made sure the slide pins were nice and greased up. Now the rotor is all rusted up and the pads aren't squeezing the rotor at all.

I've read this could be from a bad wheel bearing, but mine isn't bad. I'm wondering if my slightly warped rim might be causing the issue. Somebody also mentioned if you're light on the brakes sometimes the rear pads just glaze over

Any thoughts?

Rim shouldn't have anything to do with the caliper...you have a sticking caliper, bad master cylinder perhaps...assuming you bled the brakes then I'd go with a new caliper, or rebuilt at the very least.

did you bleed the brake?

very common problem with our TL's: seized caliper. Time to rebuild or buy a new one.

get a new caliper or rebuilt the old one (if thing has not rusted to hell).

Reading the above, you replaced both rear calipers, pads, and rotors?
Did you bleed the entire system?
Has nothing to do with wheel/hub.