Hy hotrod dad, welcome, firstly there is no auto adjust mechanism on that year of truck. The brake shoes are anchored at the bottom, not floating as modern brake shoes are, the only things I can think of is possibly a broken return spring, or the wheelcylinder pistons are partially seized in their bores, and when you stop while backing it is jamming the piston to a tighter spot in the bore from which it does not return easily, hope that helps.
