I believe the first scene it is shown is the 2nd camping trip we see "Your late...." when Ennis pulls up with the horses in the bed. According to the screenplay this is 1969 and Jack's blue Ford is a 1977. This truck of Ennis's is the same make and model, and if I could see the grille, maybe year of Jack's red truck.
This two tone combo of Ennis's truck was quite popular and there are two different 1966 Ford F100 I see regularly in my area with these colors.
Mark