Sam Rockwell really did hit his head on the truck, and punch the tree. (reference an interview at vimeo.com/859232) Previously he had gotten tips from a stunt man on how to head-bang the truck without hurting himself too much. However, when he hit the tree with his knuckles, he did it for real, and hard. He visited the hospital in the evening.
David Gordon Green, who would end up directing the film, first worked on the script for another director. After 3 years, this other director moved on to other projects and Green was approached by the producers to make the film.