Is it during cold & damp / rainy mornings that it happens?
This is one thing I never have found a remedy for (other than switching the rear drums for discs)... The rear drums (shoes) sometimes become very grabby during damp weather until they warm up some (tried three different brands of shoes with little or no change). I've just taken to riding the parking brake for a block or so to heat the brakes up and then it's fine until parked long enough for them to fully cool (gotta do this carefully too, it's easy to lock the wheels with the p-brake before they've heated up enough).
A swap to discs should eliminate the issue (I've never noticed this on my BII).
Not to divert but in case this helpful to the OP, I have had the rear brake lockup you described for a number of reasons on wet mornings and it taught me several things:
In one case it was on a 1984 Chevy truck, what happened here was that a wheel seal was leaking just a little bit, the gear oil began to cause shoe swell, I never noticed the lockup on dry days, but in rainy weather you could lock them up with even light pressure on the pedal. Replaced wheel seal, cleaned everything, no more lockup in wet conditions
Once on an S10, cheap shoes swelled in wet conditions combined with crappy wheel cylinders, would also lock up with the lightest touch of pedal in wet conditions. Swapped them, as well as crappy wheel cylinders, no more lockup in wet conditions
Have had several rangers that got locky in wet conditions, replacing the wheel cylinders always fixed this. Curious about what was going on after having dealt with the s10, I took the wheel cylinders apart on the ranger to see what was going on- they were barely starting to leak if at all, but there was a lot of corrosion in the cylinders where the pistons came out, and I suspect that the reduced traction + resistance of wheel cylinder movement caused by corrosion lead to easy lockup, the pistons would come out of the wheel cylinder and not want to go back in because of the rust. On one of these Rangers the lockup could get so bad that I had to rock the truck in 1st and reverse with quite a bit of throttle to break free again
Anyway sorry to divert just thought this info might be helpful to someone
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