There's a few great threads over at the IMDB discussion broad, especially those started and containing comments from CaseyCorneilus that seem to demonstrate many levels of symbolism in the film.
I was taken by a number of them. One that really caught my attention was contrasting the opening scene to the scene of Ennis driving away from lightning flat.
In the opening, he is riding in a truck, before dawn, moving from right to left across the screen, holding his belongings in a sack, right before his first encounter with Jack.
After the meeting with Jack's parents, he is travelling in his truck, just at dusk, moving from left to right across the screen, holding the shirts (now presumably his most prized possessions) in a sack, after his last leavetaking of Jack (visit to the parents, the homestead, the room still set up from boyhood). Closing out his interactions with Jack physically.
Another interesting one is the tracing of color in the film, Jack being identified with the color blue (which Ennis will wear on occasions of happiness) and Ennis being identified with the color tan. In the very last shot of the film, some of these symbol-watching posters, much more observant that I am myself, see that the plain out the window, although representative still of a life without the eden-mountain, is alive with three colors: the blue of the sky, the tan of the standing grain, and between them the green of the further fields. This green supposedly symbolizes Alma Jr. and the freshness of her love for Curt, arising at 19, the same age as Ennis' love for Jack bloomed. Further, it shows that although Ennis will never be able to go back to his mountain eden and have that "high" of a love again, in resolution with Jack's spirit and as a result of his relationship with Jack, there can be beauty and love of a different kind in his life - possibly his love for Alma Jr., her children, and his other daughter. Thus in Jack's death, with the affirmation "Jack, I swear . . . " is Ennis finally reconciled to his love, although with sadness and grief, and able to more fully express and participate in other love in his life.
As to the mailbox, I never saw anything convincing about the number 17 itself (I love ranchgal's analysis, tho!) but to many posters it seemed to indicate that Ennis was going to be more "permanent" and "accessible" in contrast to the fact he was not with Jack (Jack having to ask around to find Ennis after the divorce, Ennis always going to the post office to get/send mail). Thus, more settled down.
I think I'm doing a not so good job of trying to summarize what other people have noted. Hope it helps/enlightens/elicits comments and debate.