Lee, thank you for highlighting that sentence. I still get so swept up in the story when I read it that it's hard to stop and focus on one line.
And thinking now about that "treacherous, drunken light," I can see the wind that Ennis is riding against tossing the trees back and forth and sending the erractic, disorienting shadows and moonlight through the air and across the rocky ground.
The literal image is so beautiful.
Symbolically.....it's been said before (sorry, I don't remember by who) that the wind here again is Jack, Ennis feeling the force of it pushing him to return to their camp.
Not sure about the following - all you smarter folk please help me out here...
The light, that's "treacherous", difficult to use to see his way clearly....Jack's changing the way Ennis sees the world, making it harder for him to know where to go, but then Jack's also going to be the bedrock of Ennis' emotional life.