There are a number of variables involved here, one of which is the state in which you reside. Labor laws are different from state to state, and as the plaintiff in a potential lawsuit, the burden of proof for wrongful termination would be on you. This is not easy, and is usually not a quick process, so determine if you have the resources for a long legal battle and what your options might be for severance or settlement if that seems a more attractive route to take.
The second variable is that we are talking about tribal law in your case, not necessarily state law. Reservations themselves are under the legal jurisdiction of the tribe and the federal government, so consulting with a reputable attorney from your area is very important to determine which laws apply in this situation.
Lastly, you mention the possibility of defamation of character, which, be advised, is usually quite difficult to prove. If the way in which you were defamed was in public, for example, in the media, then it is easier. If this was a rumor or office hearsay, it will be more difficult. Keep in mind that defamation of character would be a lawsuit against an individual, while wrongful termination is a lawsuit against a company, so you're likely talking two different cases here.