In the last time together at the trailhead parking lot, when Jack is already in his own truck, Ennis gets really emotional and it looks like he is either having a heart attack or a fit of rage to Jack, after Jack says, "I wish I knew how to quit you."
The book-Ennis has nor verbal response when Jack says that sentence. And before Jack can get out of his truck, Ennis has straightened up, like a coat hanger putting itself back the way it was on its own, acting as though he didn't get angry in the first place.
While Ennis got into fights really easy when he was not around Jack, I don't think Annie Proulx's Ennis would have ever hit Jack when he was upset with him.
To use modern gay lingo here, Ennis tried really hard to "act like a straight man" through out the story since he just knew that people on the street could tell he was that way just by looking at him. But, only a person with the gift of gaydar would have even known what Ennis' sexual orientation was in the first place.
