I think the fact that Old man Twist mentioned to Ennis, that Jack was gonna bring him, then some rancher up to the farm, indicated that there was something going on with randall...and in the book, when Ennis hears this, it says, that he was sure then that it was the tire iron that killed Jack....."So now he knew it had been the tire iron".....
I agree though, the conversation with Jack and Ennis, was Jack trying to find out who,if anyone, Eniss had in his life....and Jack said the ranchers wife, just to let him know that he was getting his rocks of somewhere, seeing as how, he could "have sex over the phone" with Laureen.....