bdarrington,
Joe and b. j. are right, it may very well be the fuel shutdown solenoid, but like Joe said, it may actually be your starter solenoid contacts. I'm not an expert, but I have learned that the starter solenoid and fuel shutdown solenoid are in the same circuitry.
The fuel shutdown solenoid is right next to the AFC housing on the left side of the fuel pump. It has a square rubber boot on the bottom side with a shaft that attaches to the pump lever. The shaft should move upward when you go to crank and stay there while the engine is running. I've read that sometimes that shaft can get dirty and stick so you might need to open the boot to clean and lube the it.
To shut down the truck, you should be able to just pull down on that shaft after you've turned the key to the off position. If that doesn't work, open the air cleaner put your hand over the opening in the hose to the turbo, this will starve the engine of the air it needs to run... ... I know 'cause when it happened to me, I had not learned where the fuel shutdown solenoid was yet. DON'T TRY TO USE any rag or anything, as obviously this could be sucked into the turbo!
Anyway, I'd be willing to bet Joe's thought on the starter solenoid contacts is the culprit. I'm electrically challenged so I chose to just replace the starter (NAPA had it way cheaper than anyone) and my problem has been solved for almost a year now. Now, maybe in my case what prompted me to just replace the starter first was the fact that I could here an audible sound that seemed to come from the starter area. I knew it wasn't that the starter was still trying to engage, but it was making some sort of noise. Hope this helps.