Ranger fits better if you want my opinion. Marshal sounds like a commander of a whole army rather than a group of post-apocalyptic rangers.

What you could probably do with the Nazis is make them a bunch of ragtag neo-Nazi anarchist punks and raiders rather than fully trying to emulate the Third Reich like in the Metro games. And perhaps a communistic faction would be more of a faction within one of the larger factions. Perhaps some revolutionary Lenin wannabes are trying to stir up trouble in one of the more populated settlements.

I'd imagine the big factions would take most of their inspiration from pre-apocalypse DC area politics. Mostly the Republicans and Democrats. Maybe throw in some Westboro Baptist Church types, some Objectivists, and some really fucked up cults in there for good measure as well. Your only limit here is your imagination.