Proulx sticks to the basics here. Her author's voice can see all and know all. All the better to let us soar through the Wyoming skies to zero in on whatever she needs us to, right? Most of the time, she sticks with Ennis, who has the advantage of being the only member of the central couple still alive at the end. If this were always true, we'd call this third-person limited.
Occasionally she shifts the view, though, as when Joe Aguirre "watched them through his 10x42 binoculars for ten minutes one day, waiting until they'd buttoned up their jeans," (30) or when Jack "neglected to add that the foreman had leaned back in his squeaky wooden tilt chair, said, Twist, you guys wasn't gettin paid to leave the dogs baby-sit the sheep while you stemmed the rose, and declined to rehire him" (71). These moments deliver key information that Ennis can't possibly be privy to. They move the story forward, without introducing too broad a perspective.
But just as often, she'll deliberately limit her point of view to Ennis, to better stun us with her skillful dramatic effect. For instance, we never see Jack getting killed, only getting his postcard "back stamped DECEASED" (123). It preserves a sense of mystery and lets us connect more deeply with Ennis in the last few paragraphs, as he gets to the bottom of Jack's accident-that-really-wasn't-an-accident-at-all. That's the benefit of an omniscient narrator: it lets you change the rules and direct the reader's inner eye to wherever you darn well please.