Thumping directly on the drum with a 8# sledge hammer shouldn't hurt anything and should help break things loose. Spraying clean water inside the drum might help also and it won't ruin the shoes like an oil based penetrant would. Rocking the truck back and forth with another big truck or tractor might help also...make sure both rear wheels are used so you get the most ground contact/friction.
Been there, done that!
Mike B
