Mike,
Most brake drums will have an adjustment wheel inside the drum, accessible from a slot behind the wheel. You must let the brake shoes in for the drum to loosen. After you do that, it should pull right off.
Reference [http://www.ifixit.com/Answers/View/36793/How+to+repair+rear+drum+brakes|this prior question] for a guide and general diagram on how it works.
Let us know,
Frank