Is the truck driver sinful if his employer asks him to transport stolen sand, and what should he do in this situation?

You are not permitted to obey the truck owner by transporting stolen sand, because this constitutes cooperation in sin and aggression. Allah Almighty says: "And cooperate in righteousness and piety, but do not cooperate in sin and aggression." If you do this, then you are directly involved in the theft and are responsible for what you have stolen.