Here is the opinion in Chemehuevi Indian Tribe v. McMahon.

An excerpt:

It is undisputed that the Sheriff cannot enforce regulatory traffic laws in “Indian country.” See 18 U.S.C. § 1162; 28 U.S.C. § 1360. “Indian country” includes, but is not limited to, land within the boundaries of a reservation. 18 U.S.C. § 1151. The issues for decision today are (1) whether the individual Tribe members and the Tribe can challenge the citations through a 42 U.S.C. § 1983 action; and, if so, (2) whether Section 36 is Indian country. We hold that the individual plaintiffs, but not the Tribe, can challenge the citations under § 1983. And, we conclude that all the citations occurred within Indian country. We therefore vacate the district court’s judgment dismissing the complaint as to the individuals but affirm the judgment as to the Tribe.

Briefs here.