I have a 2002 F250. The Cruise Control recently quit. I checked the fuses, there isn't one for cruise control and I don't find anything else that may possibly be on the same circuit that doesn't work....
Check the switch for the brake lights. It may by damaged or misaligned. Even if your brake lights are working correctly, there may be a second switch for the cruise that is stuck or being triggered all the time making the cruise react like you are stepping on the brakes and always disengaging.