[rickpilgrim]

I don't know if the 7.4 is the same, but on the 5.7 when you are cranking the truck to start the injector wiring circuit is completed with the ignition switch and when it starts and you release the key the injector circuit is completed by the oil pressure switch. If you have an unplugged or defective oil sender it will do what yours does on the 5.7 .
It is also possible the ignition switch is out of position or was going bad and just happen to fail at this moment also. Check all your fuses and relay's first though, it could be that simple