This is actually a bit of a flaw with the newer ford auto hub 4x4 rotors. Unlike older setups, there isn't any disassembly of the hub required. However, the newer "top hat" style rotors have a large flat surface that mounts against the hub assembly where rust can accumulate, causing the rotor to really stick. This is usually more of a problem on the full size models (e.g. F-150) but is also comon on the ranger and explorers.
first, be sure the caliper mounting bracket has been removed.
try spraying rust penetrant near the center of the hub and in each lug stud to try to free up the rust a bit.
from here it will require varying degrees of brute force. If you are not planning on keeping the rotor, then beat the rotor face with a sledge or air hammer until the rotor comes off. If you want to keep it, try first hammering at the center of the rotor between each stud to try to break the rust loose or use a plastic dead-blow sledge on the face which will not cause any dents. When I worked in the shop I even had a customer run over to the car to stop me because of how hard I was hitting it! Really, it some times is the only way and often the rotor can't be saved.
To prevent this in the future, put anti-sieze grease on the hub face before putting the new rotor on. I wish ford would have done this in the first place.
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