Coyote Theory Ending (SPOILERS)
Bottom-line upfront: The answer lies in the coyotes--of course, this is still a theory. The physical appearances of the coyotes (foreshadowing by noise early in the movie) coincide with A) Carter's burial hole and B) unknown meat foraged by Mitchell from a coyote. Please allow me to elaborate.
Greetings, Everyone. I watched this film the moment it showed up on NetFlix and I, too, agree with the dichotomy of the movie's ending. Could the ending represent the effects/symptoms of PTSD as they affect the character(s) a la Anthony Swofford's (Jake Gyllenhaal) quote at the end of Jarhead "We are still in the desert"? Or did the characters in fact die and have a Mulholland Drive-esque post-death imagined scenario? One could argue that there's evidence for both, such as Mitchell's (Josh Duhamel) injured leg periodically becoming usable again and the abilities/events conducted by each character without several days' worth of water.
However, I posit another layer of the interpretation that both characters died in the desert. If Carter (Dan Fogler) died after he and Mitchell physically fought and Mitchell went through with his plan to bury Carter, then Mitchell sleeping in the back of the truck and waking up alone and scared takes on a whole new meaning. He possibly convinced himself/imagined that Carter, alive, walked away and abandoned him. But in reality, if Mitchell buried the dead Carter in the hole, wouldn't that draw the attention of the hungry coyotes foreshadowed earlier in the movie? When Mitchell wakes up and turns on the truck headlights, the coyote is facing the direction toward the hole containing a dead Carter.
Now, Mitchell continues to interact with the hole-bound Carter (possibly by this point a mauled, decomposing corpse) and perpetuates his fantasy of Carter surviving and the two setting out to find the nearest town. But during this latest string in his fantasy, reality kicks back in when he encounters the coyote eating an unknown chunk of meat. He places the meat next to him and Carter (NOTE: the meat is on Carter's side) and when "Carter wakes Mitchell up," suddenly the meat is no longer visibly next to Carter. Could this have been reality reminding Mitchell that Carter is dead and being eaten by coyotes and that he now is on the brink of death himself? Please let me know what you think.