uncleb, my "can only get worse" means if this is not a brake adjustment issue, you will have no choice but to rebuild your entire brake system. Seriously, this is your only other choice.
Next most likely cause of one set of brakes locking up is a sticky piston inside a wheel cylinder. This does happen through corrosion inside the master cylinder, hydrovac, lines and wheel cylinders. Almost always this corrosion appears as rust colored goop and hard flakes inside a wheel cylinder. This means all of your wheel cylinders are like this, along with everything else. Most of this goop appears in wheel cylinders; last place in line.
Only effective cure is cleaning, flushing out and rebuilding your entire brake system. You can fix one set of brakes but soon another will go bad, then the one you fixed will go bad.
Don't mean to hit you with bad news but this is reality. I hope you discover loosening off your brakes will fix this; rebuilding brakes on a big truck is hard work and takes almost a week.
Billyray
