Ignition switch

Try wiggling the key while the engine is running. If it shuts off you've found the culprit. I've never experienced it on a big truck, but have in my 68 camaro. (also, if you have A LOT of keys on your key ring, it can wear the switch out)

 

When you say shuts off what do you mean? Do you have to re start the truck or does it just die for a split second and come right back to life?

 

You say you have checked power and ground to the ECM, but how?

Put a sealed beam headlamp on the circuits and shakedown the wiring for the light to go out. From the ECM all the back to the batteries.

 

I had a very similar issue,drove me crazy,it was the ecm fuse in the battery box making bad connection.
I relocated the fuse to the engine bay after replacing it with a blade fuse the kind you bolt on to the end of the wires,problem solved.good luck

 

Could be your cam or crankshaft position sensor. I believe an MBN uses both. If one looses signal it will hiccup for just a second and then just use the other sensor. If it looses both it won't run. I don't think this sounds like your problem though.
I had one of the sensors go bad. When you started the truck it would cut out for just a split second and then come back to life and be fine the rest of the day