You are entitled to your opinion of the language of the characters in the story, but, having been raised in the Rocky Mountain area, I find it 'realistic'.
The short story "Brokenback Mountain" is a prize winning classic, partly because of its taut, realistic dialog. Beau L'Amour (son of Louis) calls it "great, multi-layered art," that extends the genre, which he considers mostly not relevant to modern life. Many reviewers consider it her 'masterpiece,' and I agree. Along with McMurtry ("Lonesome Dove"), Proulx currently is the best writer on the west. Charlie Baum's idea of a "ballad brought down to date" is not far off the mark.