I love the post-divorce scene, so I want to figure out some way to defend it (aside from loving the acting, the way everything that's really going on is conveyed through things that aren't said).
The mention of the twelve hundred mile drive for nothing in the short story knocked me completely for a loop, too -- that was one of the things that stuck with me after reading the short story for the first time, that plus the shirts plus "the pillow sometimes wet, sometimes the sheets." What struck me in the short story, I think, was that I didn't know about it until years after it had happened, and although so little was said about it, so much was implied by that one phone call and that one drive.
So here's what it means to me. Might not be rational, because the story and the movie speaks to some part of me that is not rational. But, well, it is what it is.
I think the divorce must really have shook Ennis, and that despite being such a loner, that he felt lonely without the familiarity of Alma and the girls. That, somehow, even though the marriage had been falling apart for a long time, that there was something comforting and familiar about having a family. And when the divorce came through, Ennis was alone again, entirely alone like he hadn't been since the day he met Jack. And Jack was the one person that Ennis talked to, the one person who made the loneliness go away. So even though phone calls say too much, even though Ennis may very well not have had a phone in his new place, he called.
I can see why it became a postcard in the movie. It might be hard to believe that Jack wouldn't have said something about coming right up if they talked on the phone. It's easier to misunderstand a postcard than a phone call (though it's certainly possible to misunderstand a phone call, especially if Jack read Ennis's loneliness correctly but didn't realize how reluctant Ennis was to do anything about it). And postcards take a long time to arrive, longer than letters. (Months, though, is a bit hard to believe.)
As for why movie-Ennis would send a postcard rather than make a phone call... I don't know. Movie-Ennis is even more closed down and reserved than story-Ennis, and maybe a phone call would be simply too frightening for him. He was used to sending postcards to Jack, so maybe that was the easiest thing to do when the loneliness became too much. And although postcards don't give the same immediate comfort that talking to someone on the phone would, Ennis might have hoped that Jack would send a postcard back... that much contact might have meant a lot to Ennis. (I bet those postcards were a huge, huge thrill every time they arrived.)
Also, another idea... given the timing of Jack's visit compared to their next fishing trip, perhaps Ennis simply mentioned the divorce on the same postcard that told Jack about their meeting time and place. But instead of simply sending a postcard back saying "See you soon," Jack dropped everything and drove to Wyoming.
As for why Jack didn't stick around, and why the girls worked just fine for me as an excuse... yes, the daughters are a perfectly good excuse, and Jack could have gotten a motel room and stuck around for a couple of days. But there's a whole other unspoken conversation going on there. It seems to me as though Ennis is talking about the girls as a way of covering up the things he isn't comfortable talking about, his unspoken fears. And Jack knows what Ennis is really saying; he's mostly figured it out before Ennis watches the truck drive by, but that one truck represents everything that Ennis is afraid of. And Jack knows it. (And I disagree with the people who have said that nobody would have noticed or cared about Jack visiting. A truck with Texas plates sitting in the driveway of somebody who, as far as everyone knows, has a total of five relatives (counting Alma, the girls, and the brother and sister) and no friends? Somebody who, as far as they know, has never left the state? I bet somebody would have asked Ennis about the Texas plates within three days. No, they shouldn't have cared, but it's a small town and people get bored and nosy.)
The post-divorce scene works as a perfect gap-filler for me; it's better than anything I had imagined about it. And I think it also works, a bit, as a piece of the Motel Siesta scene in the book, which set the course for the entire rest of the relationship in a way that it doesn't in the movie. (In the movie, there's still hope that things could be different someday after "There ain't no reins on this one." After the post-divorce scene, however, Jack changes... the pornstache symbolizes it, covering up Jake's expressive smile and making Jack look older than he is. And, I don't know, it works as a turning point for me.)
Hmmm. I don't think this makes much sense. (And it's a bit disturbing that I like such an angsty scene so much, but I do.)
(I call it a twelve hundred mile drive because that's how it's described in the story. Don't know where Annie Proulx got the extra 283 miles. Can't imagine Jack taking any detours on that particular drive. Maybe she liked the sound of the words.)