You know one plot hole is did Ennis *consider* trying to see if JAck was back on Brokeback the following summer or did he give that up? I mean not to actually work for Aquirre but maybe I dunno...sneak up there one day to see who was tending the sheep? I would think it would cross his mind.
Toadily, interesting that both of us were thinking about the same thing...
I think there was a weak reference to Ennis thinking about Brokeback a year after being with Jack. When Ennis is shown laying asphalt on a road the next summer after Brokeback, you see him looking wistfully out and beyond (almost absent-mindedly, as if he was present somewhere else) right before this scene ends. This is not long before they show Jack going back and being refused a job by Agguire.
I think it was in keeping with Ennis' character that he did not attempt to go back to Brokeback. Jack was always the instigator. Notice that it is Jack who guides him and leads him on in the first and second tent/love scenes (especially the second one.) Because of Ennis' repression, he had to be guided to where he wanted to go, as far as love was concerned. This matches Ennis' comment when both of them were in bed at the Motel Siesta. Let me parapharase as it was in the movie:
Ennis: I almost gave up on seeing you again...
Jack: Nahhhh.....
Ennis: Thought you were sore from that punch...
He was actually waiting for Jack all these years!!!!
Finally, we should not forget the simplest explanation. Ennis was already married one year after being with Jack on Brokeback. Therefore, he could not afford to be away from his wife for months on end, especially because she was pregnant that summer.