Oh yeah I will throw this in here to because I hadn't read anything on it either. To me one of the saddest bookend or mirror is everytime I see it I notice that after E puts the 17 on the mailbox he stands back to look at it to make sure its okay exactly like he was looking at the tent that wasn't right that he put up for J. To me that is still incredibly sad even after watching over and over and over.
Oooh Marlb42 this is just FANTASTIC!! Thank you for pointing this out this similarity in behaviour, I had never noticed. I think you have just solved the mystery of the significance of number 17.
Many possible meanings have been given, one of which being that this is the number of letters of Brokeback Mountain. Now look: he has made a little shrine to Jack with the shirts and the postcard. It has been mentioned before that his comfy seat is turned towards the closet, which means that he probably often sits looking at the shirts, perhaps talking to Jack. In his mind, he is now living with Jack. After Juionr's visit he makes a vow to him, (I swea), presumably acknowledging and recognising the reality of their love.
So number 17 is THEIR address, he lives with the memory of Jack in No 17 trailer, Brokeback Mountain Trailer, a sad, sad reconstitution of their real life on the real Mountain as represented by the shirts which had actually been there, worn by Jack alive, and the photo of the real thing. And he is doing his best to put this address exactly right, just as he tried to set up the tent for Jack exatly right.
Yep, this mail box is not meant to receive any mail at all...
OMG, a few more hours

today