There is an adjustment nut at the caliper, but this is only used for setting up a new caliper or cable.
There is an internal mechanism inside the rear caliper piston which adjusts the pad clearance every time the brake is applied and released. The piston is on a threaded rod that is acted on by a bellcrank or a cam attached to the handbrake cable.
When the footbrake is applied (I think) the piston screws itself up the rod to adjust the pad clearances, making sure that any pad wear is compensated for.
One of your calipers probably has a seized adjuster.
Jack up the back end, (chock the front wheels firmly!!!), support on axle stands and spin the back wheels by hand. Are they both free?
Now apply the handbrake. Do both wheels lock? Unless you've caught this really early, I'd guess that only one locks, and the other one has a seized adjuster. This is probably the side that wants more movement in the cable, the side of the T-piece that is going further forward when you apply the handbrake.
Now it's down to looking at the caliper to find out what has jammed. If you're lucky it's the external bits and WD40 and a bit of swearing will fix it. That was my experience years ago with a MkII Golf GTI. Careful what you do here, the actuating rod has a seal where it enters the caliper to stop brake fluid getting out.
If the external bits move freely then it's a caliper off job and disasssemble somewhere clean to see what's up.
If you go for a secondhand caliper, check that it hasn't got the same
problem, er, somehow...
