In the strictest interpretation of Annie Proulx's original short story with emphasis on how it was published in books, the story is set in the state of Wyoming and not just in one particular location.
She used a contrast of opposites even when Ennis grew up in the SW corner of Wyoming, in Sage, and Jack Twist grew up at Lightning Flat (used to have real post office) in Crook County in exteme NE Wyoming.
Sort of OT here, since Annie Proulx states that Ennis "ate at that place in Dubois" after they got paid in 1963, her Signal and Brokeback Mountain must have been somewhere west of Dubois which is NW of Riverton which is in Fremont County. Since Annie Proulx's Signal is not in Fremont County, it must have been in Teton County. (Even the 2nd linked article mentions that Brokeback Mountain must be in the Tetons Mountains which are in the county of the same name.)
In the book when Ennis is looking for a Brokeback Mountain postcard at Higgins' gift shop in Signal (that's near where the Stoutamire ranch was located and where AP's K. E., Ennis' brother, lived in the spring of 1963), he says that BbM is north of Signal and not over in Fremont County.
Signal and Brokeback Mountain are fictional places; but, almost all of the other location names can be found on a Wyoming map.
The screenplay writers and the movie people used artistic license in their adaptation of Annie Proulx's story.