If it's catching on the rust ridge, it probably won't work. Turn the adjuster all the way back like squeeze said, it should come off. There could also be some rust interference on the hub itself. Give it a shot of penetrant oil, like B'Laster or Kroil, and lightly rap the drum at the outside edge with a hammer, preferrably brass. That will free it up, and will also "walk" the drum over the shoes. You migth also lightly pry the drum once you get enough space to get a large screwdriver or small prybar in between the drum and backing plate. Key word is "Lightly", just enough force that when you rap it with the hammer, the drum moves off.
