I'd avoid the res if at all possible. Its a gamble and you'll have virtually no legal rights to dispute or defend your position in the event you run into trouble. They don't joke around with trespassing and show absolutely no leeway or exceptions in the event you're found riding on a road you're not supposed to be. And, accident %s go WAY up when you're on the res and many are nasty. Drinking and driving is a massive problem on many reservations.
I too wanted to ride some of the roads and areas located on a reservation in AZ and was told "you'll be arrested, your bike will be impounded, and you'll be severely fined for trespassing among other things". I went as far as speaking with the chief of police and public safety for one of the reservations and politely asked for permission...I was told the same thing. That's my .02 on the res...its just not worth it, which is really too bad. Its ironic because they can use BLM land w/ no restrictions.