I know the answer

He didn't die from the accident, that is for sure.
He was beaten to death. Remembr when Ennis called Jack's wife, the pictures show that Jack was beaten by a group of people. Remember jack is from Texas, a very
conservative place in US. During the middle of picture, Jack recalled when he was young, his father brought him to the mountain and saw a dead man's body.
That is an implication to Jack's fate.
Jack also said several times during the movie he can't stand only once a year to have a sex with Ennis, so he went to Mexico Broader to find a partner several times. I think he was beaten to death because some local people find his behavior and then beat him to death. Of course, his father-in-law could be an explanation too.