Riverton, in Fremont County, is more or less in the Middle of Wyoming (slightly to the West, somewhat); but, Lightning Flat, where Jack is from, is in the extreme NE corner of the state in Crook Country. (Lightning Flat used to have a post office and it was in NW Crook County a the Wyoming-Montana state line.)
Jack probably went through Denver on his way up to his folks from Childress, Texas. The US Highway between Denver and almost Crook county is # 85. By taking that route, he still wouldn't get within 180+ miles of Riverton.
So, since the book Ennis told Jack that his brother, K. E., lived in Signal when both of the guys worked together in 1963, I say that Jack might have known where Ennis's brother lived and contacted him. And, it might have been possible that K. E. told or wrote Jack that Ennis had moved to Riverton but he did not have his address.
While we, as observers, can say "Jack was Ennis's lover," Ennis himself would never admit to that. He could only think of Jack as a buddy and nothing more than that. (Oh, ramblin' here . . . I would say that the reason the Movie Ennis said that Jack was his fishin' buddy to Alma was that Ennis did go fishing before Jack showed up in 1964, or he had already planned to do that.)
Oh, by the book again, Ennis had not been married to Alma 4 years yet when Jack showed up. It was more like 3 1/2 years.