i've been watching this scene over and over again trying to make out why when ennis is talking to jack, he stops and looks sideways and there's a white truck passing by on the other side. When he sees him looking that way, jack looks too...and then he looks back at ennis....and they have a sort of meaningful look in their eyes...Then he just nods and leaves....Does that white truck mean anything?.....please someone help me make some sense out of it....i'm going crazy thinking about it and trying to work it out!!!! 
Thanks (

) to his homophobic father and his past, Ennis's number one fear is being outed, being thought "queer" by others. ("Do you ever get the feeling that everybody knows?") Any time he was with Jack and anyone else was around (he's already feeling self-consious because his daughters have withessed a hug--which Ennis keeps "just a hug."
The extra twist here comes from the fact that Jack says he has asked "five or ten" (?) people around town where Ennis has moved to. The idea that Jack (who seems to ping a little on folks' gaydar--see the rodeo clown, the murderers, Aguirre) has been asking (rather fervently) around town for Ennis makes Ennis very nervous. That's why he doesn't want the person in the truck to see, wonders whethere the guy in the truck is on a spying mission, etc.
Perhaps this stirred-up fear in Ennis is the cause for his brutally cold response--not tellling Jack to wait a few hours, not telling him to come back Monday, just SENDING HIM AWAY. I've said it before; I'll say it again: Ennis needs to be slapped up the side of the head for this. Very bad form.
But it's because of who Ennis is, as his past has shaped him, that he is worried about the truck and worried in general about folks finding out about his relationship with Jack.