It's interesting to note from the BBM script that Ennis only takes two shirts with him up onto Brokeback - when he gets out of the truck at the start of the film, the script says "no suitcase, just a grocery sack stuffed with his only other shirt..." As you say Amanda, if the shirt Alma's washing at the ranch when the girls are small is the shirt Ennis wore in TS1, the one in the "eternal Dozy Embrace" would have to be the one from TS2.
That's also interesting though in the choice of shirts (one from TS1 and the other from TS2) in the "eternal Dozy Embrace" because whenever you read anything about the making of BBM, everyone involved always says that Ang Lee had things set up in very precise ways for a reason, and there was a comment in the Pierre Tremblay interview about even the wardrobe being chosen very specifically. I wonder if that extends to those particular two shirts?