I had what I thought was a sticking caliper and it turned out that my master cylinder was on the way out and not fully releasing pressure to the rear brakes causing the drag.
If your brakes drag and over heat they will either warp the rotor, or pad material can adhere to the rotor causing a high spot that hits when the wheel rotates (that happened to me). Do you also feel a pulsing when you brake?

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1985 300D 170k