Me and my buddies went offroading last night, and I tried going up a prety steep hill and the truck stalled out. I figured that all of the fuel sloshed to the back of the tank and the fuel pump sucked a little air through the lines. I cranked it over till the batteries died, then we towed it and I caught it in gear and it fired right up. Am I right in my assumption? Is there anything to fix this? It's kind of embarassing when the badass diesel can't make it up a hill, but a wimpy ranger can!

Was the tank too low?

There was a half tank in it.

Lift pump is probably dying or dead, and IP couldn't pull the fuel up a hill - do the Diagnostics Checklist, go from there.