Does anyone know the specific law prohibiting a repossession off of an Indian Reservation without a replevin or voluntary arrangement?

A vehicle belonging to a Native American who resides on a reservation cannot be repossessed if 60% or more has been paid on the balance of the loan. Relinquishment or repossesson of any property belonging to a NA on reservation land, has to be voluntarily submitted in written form by the purchaser or by an order from the Tribal Court. Tribal Codes will vary slightly, but the basic codes apply on all NA reservations. See link to this answer page