Gene,
If the Alt turns out OK you might have a solenoid issue...
Your starter solenoid has a wire going from it (terminal "R") to the coil or coil side of the ballast resistor. This wire puts full battery voltage to the coil when starting the engine. If the contact in the solenoid is shorted to the Battery + terminal it will provide voltage when the switch is off. The "S" terminal on the solenoid comes from the ignition switch to energize the starter solenoid and start the starter. I'd start the truck and turn the key off and see if you have voltage with a test light on terminal "R" at the starter solenoid. If you do remove that wire and see if the truck stops. If it stops you have a bad solenoid...
What steps are you doing to actually turn the truck off since the key won't do it?
Mike B
