First big thing... have someone tap the brake pedal while you watch the brake lights. If your brake pedal switch is non-functioning/ malfunctioning, it will affect the operation of your cruise control.
Is your MIL (Check Engine) light on? Or is your Overdrive Off light on constantly/ flashing? If so, you've got a Diagnostic Trouble Code (DTC) set somewhere that might be an indication of a problem. For example, VSS (Vehicle Speed Sensor), TPS (Throttle Position Sensor)... those could affect your cruise control operation as well.
I just looked on the Ford Service website and it says that according to your symptoms (Light flashes once after you press the SET button, no "dynamic pull" meaning your truck doesn't accellerate briefly then settle into speed, and no function of speed control) it's most likely the control cable or the servo motor.
It says to check the speed control cable for proper connection at both ends, then to check and make sure the cable isn't kinked or binding. If the cable operates freely and is connected properly, it's most likely the servo motor itself.