Hey CH:
Don't know 'bout rdx, but I've thought a lot about the geography. DuBois, Wyoming is a real locale easily found using Google and there's even a real "place" in DuBois where I imagine they ate, here it is in the 1960s, the Cowboy Cafe:

But 'course I'm only guessin'. DuBois is on US 287 on the way into Yellowstone National Park, now crowded in summer 'cause of the Park, but very nice rest of the year and seems almost deserted in winter.
Ain't no signal. Nowhere, least not in Wyoming. An imaginary place dreamt up by Annie. Obviously somewhere up in the mountains, but it's not clear what range, Bridger-Tetons, Wind River, Big Horn, etc. Like Brokeback itself. There's not many highways into/out of DuBois and the nearest "town" is Burris which is about 25 miles. Hardly nuthin' in Burris though, in the movie anyway, Signal is much bigger than Burris. My conclusion is that it really is just imaginary, though I've never seen any conclusive info on this from the mouth of AP. If I recall correctly she was living in and about central WY at the time, I believe she cited her frequent trips driving past a certain cement plant in Casper (I think) when she was working on the Dozy Embrace scene. Signal could have been a compilation of numerous small Wyo towns.
You pose an interesting question as to if Ennis went to eat alone in DuBois or with Jack. I'd always imagined they went together after coming back down off the mountain, sort of a last supper thing. But there's really nothing in the narrative that would indicate it couldn't have been after they were paid and split up and then Ennis went to eat alone, which in some ways would be more in character.
I've always interpreted Ennis's fist smashing in the ally as his striking out at himself and the frustration he felt at falling in love with a man. A man now gone, and he knows not where. The innocent bystander just happened to be in the wrong place, wrong time, just like so many of Ennis's "victims". Ennis didn't especially go lookin' for trouble, but he wouldn't shy away from it either. He "fixed it" if he could, just like with his brother KE.
JMHO.

Hi Jackster,
Thanks for your informative reply. I'd forgotten Signal was "some pretend place" so obviously there's no point in trying to work out how far it is from
Dubois. By the way, I loved the snapshot of the Cowboy's Cafe in Dubois. It looks exactly like the sort of place Jack and Ennis might have dropped into for a bite to eat.
I've now come to feel that in the short story's arrangement of events, there is no time for Ennis to go off to Dubois by himself for a meal after parting with Jack and then fall ill while driving on somewhere else. In the short story, it seems to me, the sequence of events must be that they come down from Brokeback, go together to Dubois for a meal, and then return to Aguirre's trailer where they part. The puking takes place very shortly after the parting so, if the meal was the cause, it had to be an hour or two back:
" 'Right," said Jack, and they shook hands, hit each other on the shoulder, then there was forty feet of distance between them and nothing to do but drive away in opposite directions. Within a mile Ennis felt like someone was pulling his guts out hand over hand a yard at a time. He stopped at the side of the road but nothing came up."
The movie's version, however, is somewhat more straightforward as Dubois and a meal there is never mentioned either when they are parting or when they are talking in the motel. In the movie we have them parting in a noticeably more subdued manner than in the short story - they never even touch each other - and then Ennis walks away. However, not long after that - no doubt "within a mile" - Ennis steps aside to puke or at least try to puke. Most viewers , I believe, see what is happening to Ennis here as an outward manifestation of his distress at having to leave Jack. Certainly, in this version, no "bad meal" is on hand to provide another possible explanation.
However, how clear Ennis is about what is really upsetting him is difficult to say and so is left up to each individual to ponder. In the short story Ennis says it took him "about a year a figure out it was that I shouldn't have let you out a my sights" and maybe this is also true of movie Ennis as well. If this is the case, then what we are witnessing in that alley is an inchoate explosion of conflicting emotions of love,anger, desire, frustration, fear, even though none of these Ennis fully comprehends. No wonder he yells at the innocent bystander "what the f--- you lookin' at?" when Ennis barely understands himself.