"Where Should We File a Claim that Arises on the Reservation?"
Answer: It depends. Subject matter jurisdiction of tribal, state or federal courts depends largely upon (1) whether the defendant is an Indian or non- Indian person or entity; and (2) whether the act occurred on Indian fee or allotted lands, non-Indian-owned reservation lands, or even a state right-of-way on the reservation. These two complex issues should be the first area of inquiry for any question regarding civil jurisdiction over a dispute arising on a reservation.
State courts have jurisdiction over lawsuits between non-Indians arising on the reservation. However, jurisdiction over a suit by any party--Indian or non-Indian--against an Indian person, a tribe, or tribal entity for a claim arising on the reservation, lies in tribal court. So, if your client is prepared to show clear or unequivocal waiver of immunity, you should file any tort claims against the tribe that arose on Indian lands or in tribal casinos, in tribal court.
In particular, state courts have jurisdiction over any dispute arising from an auto accident occurring on a state right-of-way through the reservation, including a dispute between non-Indian citizens, and a suit by an Indian against a non-Indian. As such, common claims that arise on Idaho state highways running through reservations should be brought in state court.