I think I read a thread once about a guy that had less that 1/4 tank of fuel and left his truck parked on a hill just to find out it wouldn't start when he returned. I think it started after some cranking & returning it to level ground. I'm not even sure where to tell you to start searching for this thread, but I know it was on here. There was some discussion about the fact that Ford was aware of this problem with the vehicles, but wasn't gonna fix them due to it not being a saftey issue.
Have you tried to start your truck on level ground? Or add a few gallons of gas (enough to make a difference...like 5)
06 Screw XLT, 5.4L, 4x4