Hi there,
Just a couple of thoughts,
If the wheel is locking, particularly in the forward direction and not in the reverse, take a good close look at the top of the lining on the primary shoe, and the bottom of the lining on the secondary shoe. You could just have a lining that has come loose from the web, be it loose rivets or a bonded lining that is losing its bond. Give the linings a good solid push with your fingers as though you were peeling them off. If they are loose, it should show. If one comes off, then better now than on the road. Poor bonding and loosely riveted linings will cause this condition. I have had it happen on my own truck and many customers as well.
Hope it helps.
