As I understand it, when you first turn the key to "run", the pump turns on for a short bit to make sure the fuel pressure is up to spec, then it shuts off. When you turn to "start", the fuel pump should get full voltage. After the engine has started, the fuel pump should get full voltage until you kill the ignition.
If you're screwing around with checking voltage, you can disconnect the wire that engages the starter and then you're free to have someone hold the key to "start" without wasting your battery on cranking the truck indefinitely.