Sheriff Joe can still begin pulling over Federal Vehicles and impounding them and siezing them under CURRENT Civil Asset Forfeiture laws. Just claim they are using the vehicles in a conspiracy to violate the immigration laws of the United States. Never arrest or charge them, they really have no chance of getting their property back, just like Ordinary Citizens.
I am a supporter of Sheriff Joe and his willingness to stand up against this de facto amnesty. However, as a state empowered official he has zero legal authority when it comes to federal property. Please remember that the federal government is still a separate sovereign from the state of Arizona.