There is a part in the book that concludes that Ennis believes without doubt that it was the tire iron. When Jack's father says on page 282 of Open Range, 'But like most a Jack's ideas, it never come to pass.' So now he (Ennis) knew it had been the tire iron." But how he knew isn't clear.
I believe that Ennis' fear from the childhood experience of seeing poor Earle murdered was so deep seated that he could never overcome it and it led him to believe "It had been the tire iron", but he had no real way of knowing for sure which is why the story ends with, "There was some open space between what he knew and what he tried to believe, but nothing could be done about it.."
The scene of Jack being murdered was a flash in Ennis' imagination, a flash fed by that deep seated fear. I also felt the Lureen believed what she has been told: that Jack was killed by the exploding tire. Her difficulty when speaking with Ennis arises only when she realizes that Ennis and Jack herded sheep on Brokeback Mt where Jack wanted his ashes left. Her anger toward Ennis is the realization that he, not she, was closer to Jack. She is also hurt by this fact which is why she hung up on him so abruptly.
What do I believe about how Jack died? I don't think there is any way to really know; all I know is it's the saddest damn thing that could have happened, but the accident and the murder are both real possibilities and I miss Jack like the devil misses heaven.
Rayn