I think Ennis was pretty close to tears here. That seems to be the case whenever he's shooting Jack's airplane out of the sky, actually - it's the same when he says "ain't no reins on this one" /b] :\'(
You know, I didn't realize this until something I read in one of the fanfics made me think of it ... but really, we see Jack cry after the divorce debacle, when Ennis turns him away, but even though we (usually) consider Jack to be the more openly emotional one, it's Ennis we see in tears or close to tears more often. Instances I can think of:
Leaving Brokeback he breaks down in the alley;
in this motel scene he seems close to tears;
as you say, NoReins, he looks close to tears when he says "ain't no reins ...",
of course, breaking down at the lake that last time;
when he's talking to Lureen on the phone, seems like he's fighting tears;
at Jack's parents house, both as he's looking around the room (tears) and finding the shirts (many, many tears);
And that last "Jack, I swear .. "
Sorry if that's OT ... but I hadn't really noticed his tears in this motel scene until this thread was started, and now it's got me seeing tears everywhere ...