I'm happy that folks are enjoying this idea for a thread!

One of Meryl's observations about the brown truck in the postcard scene that I found so spooky is that the angle on the truck is such that you can't see a visible driver! It really is as if a ghost is driving it... at least in the still provided here.
Yeah, the color of trucks and the changes in trucks across the span of their relationship seems to be a major theme or visual element, perhaps meant to be clues to deeper meaning. The trucks are usually quite striking one way or another, and I think it's kind of fun to think about both Jack and Ennis's trucks in relation to one another.
Buffymon, I like your analysis.

To me, Jack's first truck is almost comic in it's banged-up and run-down state... and of course we're immediately made aware of it by the funny tire that spins causing Jack to kick the truck. By the way, I've always thought that this might be foreshadowing about Jack's death (even this early on).
Then, later during the early years of his marriage, Ennis gets that clunky black truck that seems somewhat visually similar to Jack's first one. I always wonder if that's meant to kind of indicate that Ennis is thinking about Jack during that phase of time. And, I also wonder if it may be part of the idea that Jack is one step ahead of Ennis, but Ennis comes along eventually. At first Jack has the clunky black truck and Ennis walks. Then Ennis has a clunky black truck, and Jack moves on to a new truck.
That red truck during the reunion seems so important, and the image of that happy red truck driving off to the mountains is such an iconic BBM image.

Yeah, and there's certainly a lot ot be said about the later blue and aqua blue trucks.