You need to get him to agree which vehicles are yours in mediation. If that does not work, you will need to file for temporary orders to keep him from taking your vehicles.
Understand also when you leave mediation you will not be divorced, you will still be married until the judge signs your divorce decree.
If you hide a vehicle, that is concealing property and in violation of the Preliminary Injunction (which is a Court order) and you will be in contempt.
If you need help with any of this, contact me at sw_legal@hotmail.com or call me Monday at 602-488-5490.