I put a few tricks on a friends jeep before he took it to Mexico for a year. I installed a toggle switch on the dash to disable the electric fuel pump. I also used a relay so you had to have the headlights on high beam before the starter motor would turn.
One I've thought about but never done was to use several identical toggle switches as a "combination lock" for the starter or ignition circuit. You could wire them so some had to be up and some down to complete the circuit. Most thieves wouldn't take the time to try and flip lots of switches different ways.
A manual valve in the fuel line would work too. You would just turn it off when you left the truck unattended. If it were electrically activated, even better.