Some states do have some sort of law concerning animals riding in the pick up bed. My husband goes to great lengths to insure our pups are safe riding back there. He installs a piece of round pipe about the thickness of a cigar, that runs the length of his toolboxes. There he attaches rings and a short lead to hook the dogs up to. They can't jump out or climb up on the boxes. He even has a "beware of dog" sign posted, so lookie lou's don't get to close.
They still can wander in the bed of the truck and lay down to get out of the wind. We have a rubber bed mat so they don't slip and slide all over, and a piece of carpeting to lay on if the rubber mat is to hot.