Both the story and the screenplay leave us with uncertainty about the manner of Jack's death. However in the 2004 shooting script there is a scene that seems to rule out an accident:
EXT. GAS STATION -- ROAD OUTSIDE CHILDRESS -- DAY -- 1982
Jack's truck pulls up to the dirt lot next to the gas station. A mechanic, tire jack in hand, fiddling with a car, takes a beer from his buddy, who sits on a tire nearby. They both watch as Randall gets out of the truck and walks to his own truck parked in the lot, waving back at Jack.
The mechanic trades glances with his friend.
Their pov:
Randall's truck pulls out of the lot, goes in one direction.
Jack's pulls out after him, going in the opposite direction. wide:
We hold on Jack's truck, as it drives off into the distance.
[The unnumbered scene occurs between Cassie's kiss-off (147) and Ennis receiving the undelivered postcard (148).]
=aside= Players
Off for a couple of days, NOT fishing.