I think the problem that you and Ruthlessly were struggling with, Mikaela, comes from the change from the short story to the movie. (This movie is really exceptional in that its translation to the screen left few holes in the logic, and may, in the minds of many (*hi Katherine*) have resolved problems in the story.)
So in the story, Ennis actually calls Jack, on the telephone, to tell about the divorce. And that's just
huge. That kind of change in Ennis's typical behavior could very logically explain why Jack would think everything had changed.
But the movie changed that phone call to a postcard. And yeah, that moves the communication into the realm of typical Ennis behavior. And yeah, it does suggest that Jack really didn't understand Ennis very well. Which conflicts with my impression of the characters elsewhere in the movie. (Though I guess Jack misjudges Ennis's reaction to being told that the summer is ending early, there on the mountain, so maybe I'm badly overestimating Jack.)
Edit: I kind of like the idea that, somehow, Ennis let a bit of his loneliness out onto that postcard. I don't know what he could or would have said, though. It might not have taken much to have gotten Jack's hopes up.
Edit again... But I really, really like the idea of Ennis calling Jack, just that one time, and both of them being so thrilled at hearing each other's voice that a little of the energy of the reunion scene infused the call. (The
emotional energy, of course.
) Though... well, I'm projecting. I've never been much of one to use the phone myself, but those few phone conversations I had with the other half of a long-distance relationship... yeah, I would have driven 1200 miles with no other invitation after them.