I would not recomend leaving any animal in the back of a vehicle. U-hauls are notoriously unreliable and if not maintained properly exhaust could seap into the container and carbon-monoxide poison the animal in the back. Plus there is no way to secure the cariers in the back of the truck and pieces of furniature could fall on your cats and seriously hurt them. If you don't have the room I would sugest accompanying them on a bus or renting a car to take your pets.