The most common problem with the cruise control is the brake pressure switch.
There has also been a TSB released about the BOO (Brake on / off ) switch input malfunctioning due to a probme in the BRAKE SHIFT INTERLOCK SWITCH.
The BRAKE PRESSURE switch is on the master cylinder. It's an easy part to replace. If you have a DVOM or a testlight I can tell you how to test it.
You need a scantool to do the brakeinterlock sw test. If you don't have one, either your regular repair shop or the dealer. Most repair shops have scantools that can test the BOO input. Tell them to monitor the BRAKE ON OFF (BOO) input while the brake is released. It should read OFF. According to the tsb, Apply the brake, move the gear selector to DRIVE. Release the brake; if BOO reads on or intermittently on while the brake pedal is released, replace the interlock switch. If it says off, move the gear selector with the brake not applied into different positions while monitoring BOO. If it ever says "on" with the pedal released, the interlock switch is faulty.
2004 F150 FX4 4x4 on Goodyear Wrangler Duratrac 33". Paint matched ARE bed cover, front end leveling kit, 3 TV's and 2 10" Kickers under the back seat and all new front end linkage (4 ball joints job the week I bought it)