Reading through the various replies on this thread has been very rewarding. There's always more insight to be had with this group of people!

I reached the conclusion some time ago that I was unable to decide with certainty exactly what the last sentence means. As much as anything, it seems to serve as a reminder that we can never truly know what's hidden in the innermost heart of another human being. Despite having come to know Ennis well through this story, despite having followed him through the fear and pain and sorrow and joy and love of those 20 years, - his thoughts are still his own and sometimes indecipherable, to some extent he's still inscrutable, still an enigma. I wonder if Annie Proulx herself had one specific meaning and one meaning only read into that last line; - however that is, it seems certain she wanted to make it ambiguous to the readers to interpret in whatever way makes the most sense to them.
What seems certain to me though is that the brief paragraph preceding it covers the whole of Jack and Ennis's time together. The beans and spoon handles of Brokeback, the plans and dreams that came to nothing, the shadow of the tire iron that loomed over their relationship the entire life because of its significance to Ennis, the love, the sex, the grief and tears. That it's cast in
"cartoon shape(s) and lurid colours that (give) the dreams a flavour of comic obscenity" seems very harsh, entirely unsentimental, and pointing to a great deal of
tragic irony in Ennis's situation at the end of the story. And *that* makes me think that if I had to choose *one* meaning for the last sentence, I think it is the following:
Ennis *knows* without a doubt, in his heart, what Jack meant to him - that he loved him. But he doesn't know that Jack ever fully knew or was made to understand how deeply Ennis loved him, - the way Ennis never spoke up, the way he behaved, from punching Jack out to omitting any verbal response to the "Sometimes I miss you so much....." to blaming Jack for making him a "noone". Ennis can only try to believe that despite his silences and fears and his constant holding back, what he did do was still enough, so that Jack *did* at one point experience the confirmation of Ennis's love for him without any doubts lingering and lurking, *did* believe all that Ennis "swears" to Jack after Jack is gone. It seems to me one of the worst regrets Ennis has to live with is that he never managed to tell Jack his feelings right out, loud and clear. Despite their relationship taking up and shaping Ennis's entire life - he never said the words, except indirectly, and as a part of a bitter accusation. Sure, he has to try to believe and hope that what he did do was somehow still enough - but there's a huge space of doubt between that and certainty.