The 'divorce' truck scene was, for me, by far the best of the MacMurtry/Ossana invented scenes. It had to be there, since it described a crucial development in Jack and Ennis's relationship, yet the original story only included it as a seemingly throwaway remark near the end, when so much else is happening. They realised it and integrated it into the film's story-line perfectly and it made for one of Jake Gyllenhaal's best moments, lurching from 'King of the Road' elation to devastation. It was also unusual in coming from Jack's perspective, since overall the movie took Ennis's - fair enough since the story was written that way and fidelity to the souce was (quite rightly) paramount. To my mind, along with the 'Reunion' scene, it virtually bookended a crucial part of the story, as marking the honeymoon period of their post-Brokeback relationship: Ennis's excitment being fulfilled in the earlier scene then Jack's destroyed in this one.
The discussion about the Jack/Ennis relationship here has been interesting and it does seem that Jack's disillusionment was real, though it did take a few more years to finally surge through, with a crash, during the final encounter between them. I think this is what we see in the contrast between the two images of Gyllenhaal's face when he gazes at the departing Ennis - adoringly in the perfect flashback, but haunted and desperate in the later image that immediately follows it. Jack has made a decision, and soon afterwards, when visiting his parents, he mentions a new candidate for moving up with him. Tragically, pursuing this may have led to his death.
