It's 2, maybe 3 a.m. and I'm in the passenger seat of Sam Lewis' old brown van as we hurtle toward Terlingua. We wander across the center stripe and back and back again - Sam's not near as worried about it as I am. Sensing my discomfort, Sam says, ''Oh, I just kind of aim this thing down the road.'' And he does, not steering as much as giving the steering wheel a slap every now and then when we appear to be on the verge of disaster. I'm reassured by the thought that Sam might have a few armadillos...