I caught the last part of the movie on HBO tonight, and I think I noticed one more yin-yang thing. Check out the door of the trailer in the last scene:
The dark half should have a white spot, but instead there's a white spot on the light half, as if Jack's essence is now residing in Ennis.
Next to the dark half, a shovel rests against the wall, symbolic of Jack's death. It also reminds me of the idea that Ennis is "buried" in the trailer, too, along with his sacred relics, the shirts.