I meant to reply properly to this post but I was on my phone...
Here is what I had to say in a previous post about brakes locking up. I'm not one to back off the brakes... in my experience it amplifies the problem. But yes... drum brakes are adjustable to get proper clearance.
This is my cut and paste ...
You sure there is no contamination on the drums/shoes? Drums look good... no hot spots?
These trucks are prone to locking the rear wheel(s). In part due to the rear of the vehicle being so light. There was a TSB that addressed water intrusion. Had you seal up any open points in the backing plate.
Very important that the park brake is not hung up too. With the park brake struts in place... the tops of the primary and secondary brake shoes must be fully seated on the backing plate anchor pin. If not... address the park brake adjustment so they do seat on the anchor pin fully.
I have a theory that having the rear brakes adjusted to loose will cause a rear brake to lock up...
Under normal drum brake operation... when you apply the brakes fluid is sent to the wheel cylinders and pushes each piston equally and pushes the primary and secondary shoes out to contact the drum. Rotation of the drum causes the drum brakes to self energize. Meaning the primary shoe will react to drum rotation and force the secondary shoe back to the anchor pin. The fluid that was holding out the secondary shoe is now displaced back onto the primary shoe wheel cylinder piston. So the theory is that the further amount of distance the shoes have to travel... the greater amount of fluid that gets displaced onto the primary shoe during self energization. Thus locking up the brakes.
I always favor the tighter side of the rear brake adjustment for this reason... and it has worked well for me for many years without locking up my rear brake(s). Once you get them adjusted... pumping the rear brake pedal several times while rolling backwards will keep them adjusted properly.
Here is the link to the entire thread I got my cut and paste....
Rear brake on driver side engage TOO MUCH - 1989 Ranger | The Ranger Station