I think Ennis had already figured out that there was no ranch foreman's "wife", sometime between the final confrontation and the scene with Cassie in the diner. Ennis might not have been well educated, but he wasn't stupid, he would've re-played that conversation over and over in his head and would've put the pieces together. Jack needed to have sex with men, he went all the way to Mexico for it, why would he risk "getting shot by Lureen or the husband" to have sex with a woman, when it wasn't what he really wanted or needed? (ok, getting shot is probably an exaggeration, but it still would've caused a whole lot of trouble if he'd got found out). And Ennis says "All them things I don't know..." ,plural, implies that he thought there was more to know than just Mexico, and Jack's "...if you don't ever know the rest" suggests he was right.
So I think Ennis worked out that it was the ranch foreman himself that Jack was involved with, but it wasn't until the scene with Cassie that it occurs to him that this guy could actually be a threat. Cassie makes it clear that she loved Ennis, and probably still loves him, but she eventually accepted that he couldn't give her what she needed and decided to cut her losses and move on, with the guy in the diner, who even though she says "he's nice... he even talks" you get the feeling she sees him as second choice to Ennis. Alma did much the same, stayed with Ennis for as long as she could, tried to make it work, but eventually had to admit defeat and moved on to Monroe (ok, arguably Alma "traded up" with Monroe, but I still think she would've preferred it if she could've had that life with Ennis). Ennis knew that he couldn't be what Alma or Cassie wanted him to be, because of Jack, but he couldn't be what Jack wanted him to be either, couldn't give Jack what he needed. Did it cross Ennis's mind then that maybe Jack too would get to the point where he felt he had to cut his losses and settle for second best, now that there was someone else in the frame who might be more than willing to take that second place? If Randall was willing to take the risk involved in having an affair with Jack, chances are he would also be prepared to take the risk of them living together, and after Jack's saying he wished he could quit Ennis, would Jack have turned down the chance to do just that, if Randall offered it to him? (As I've said before, I think the suggestion of Jack and Randall living together could likely have come from Randall, maybe not moving to LF, that would be a bit presumptuous, being Jack's parents' place, but at least the idea of them ranching up together somewhere could have been Randall's)
So I think all these questions going round in Ennis's head, added to the fact that he hadn't had a postcard from Jack about November, aroused an anxiety in Ennis which made him send the postcard to Jack, to remind him that he was still there, that he was looking forward to seeing him, and that if Jack was making plans that didn't involve him, to hold on until November. I think that had they met in November, Ennis would've demanded answers from Jack about Randall, and more than likely lashed out with his fists when Jack confirmed Ennis's suspicions, but Jack would've hit back, physically and verbally, giving Ennis the choice of either losing Jack for good, or finally forgetting about having to stand it, and fixing it once and for all, always and forever. I have to believe that he would've chosen the latter, if only he'd had the chance... :\'(