I had this exact issue a couple of months ago. Both rears binding, more so on the left. Mechanic had it up, handbrake cable released easily enough (phew, no stripping the interior) but the brakes had had it. Two rear discs and pads, plus one new caliper as the thread was stripped, plus one tyre as it had caused excessive wear on the harder sticking rear left.
I had all the pads replaced, plus all the calipers reconditioned. The best part of £800 later (needed ASAP for MOT retest) I was ready to cry, but there you go, at least they're mainly consumable items. The brakes after were simply immense. No squealing at all, minimal dust, clean wheels for more than 10 miles. I noticed immediately when reversing how the braking had improved. During the binding, as the pads were already on the discs, the pedal feel was odd to say the least (not much travel/'stiff' pedal but only in reverse) and required A LOT of effort to stop in reverse - maybe a handy little self diagnostic for anyone wondering if theirs are binding...