I worked on Navajo Reservation, Do I still file Federal tax?

Answer 1

Frivolous tax returns; “Native American Treaty.” This ruling emphasizes to taxpayers, promoters, and return preparers that there is no right to exemption from federal income tax for Native Americans under an unspecified “Native American Treaty.” Any return position based on an unspecified “Native American Treaty” has no merit and is frivolous. As a general rule, Native Americans are subject to federal income tax just like every other American.

You are required to file a Federal Tax return

Answer 2

Of course! You're still a US citizen and live in the US.