Wow, one day's worth of posting!
Here is throwing my hat into the mix.
babytammy7, great question, what if Jack never stole that shirt!? The movie would end so differently.
I also thought that cowboy on horseback was carved by Ennis? I thought of it right the way, when Ennis pick it up almost immediately upon entering the room. And he had tears in his eyes when he put it down. Thinking that Jack kept this little token of their time together all this time. I am fairly certain it is what Ennis was carving because the horse's posture look almost exactly the same, with its hind legs lower than the front, and the distances between the legs are in the same proportion. Only the cast and crew, and Ang Lee would know for certain. However, I would be very surprised that with all the careful attention to detail elsewhere in this move, if this little coincidence was left to chance.
I agree with lamusica that Jack would never take anything from Brokeback Mountain to Texas. He never even told Lureen about it. That's why she was caught off guard when Ennis told her they herded sheep there one summer back in '63. And of course, Ennis would have figured on that right the way. But hiding the shirts in a nook in the closet was a much more powerful message which tells Ennis that Jack "treasured these shirts" and kept them unwashed for obvious reason.
I think there is another reason why Jack stole that particular shirt, even without the blood, which Ennis himself wiped there, when his nose was bloodied. Jack wiped his own sleeve to clean Ennis' still bleeding nose, just like he did with the kerchief when Ennis fell in the encounter with the bear. And the reason is this. That shirt of Ennis' was Jack's favorite. You can see him washing it in the river in the raw, wearing only his boots, after their FNIT. The short story has this scene and poignantly described the shirt as Ennis only other shirt.
What makes the two shirts even more powerful symbols than the blood brother angle, is the fact that Jack put Ennis' shirt inside of his...just like in the motel scene when he wraps his arms around Ennis...And when Ennis found the shirts, he instinctively took the sleeves and put his arms around them, like Jack did to him...another reason why he reversed the shirts...he was able to show the same love to Jack that Jack showed to him.
And what makes me think that Annie is such a genius is the subtle meaning of finding the shirts for Ennis, not only as proof of Jack's love, but also of Jack's Ma's acceptance of them. I didn't pick up on this until the discussion in the forum. As I verbalize, or write down the words, I realized what must be true, and confirmed by Roberta Maxwell's interview.
That's why I love you all in your discussions. Thank you jackster for starting this topic.