There's a certain balance because people are moving everywhere, so one person rents a truck from Michigan to California, then someone else rents a truck from California to Minnesota, and then someone else rents a truck from Minnesota to Michigan. Because of all that, it stays pretty equal. That's why the U-Haul trucks you see are hardly ever from the state you see them driving in.
Now, if there is a big wave of people moving into an area, much more than moving out, U-Haul does have to do some redistributing of trucks, but it's not on airplanes. Are you crazy? Do you have any idea what that would cost? It's on freight trains.