Why does truck shuts off immediately after it starts?

Try this experiment, try cranking your truck and when it starts, hold on to the ignition key in the start position for a few seconds, you will hear the starter gear, spinning, so don't do it for more 2-3 seconds. If your truck stays running in the start position and dies when you let go of the key, it could be an ignition switch issue. If it dies, even in the start position, then have the computer scanned for codes. I hope that this little test helps. I do it all the time and it helps me isolate the issue.