Desert Rangers where a scout/sniper group formed by former-US military snipers that served in the Gobi campaign, after the war some trained sharpshooters found their equipment, forming the Nevada Rangers and, later on, joining the NCR, forming the Black Ranger NCR unit.
- In Fallout Tycho gives you some Ranger background - they didn't come from any one source, the were basically survivalists who formed a community in Nevada and acted as vigilantes in the absence of law and order, protecting innocents from raiders and dangerous animals and the like. Originally they were just a shoutout to Wasteland, where the PC and party were Nevada Desert Rangers. There's no connection to the Gobi campaign, the Gobi Scout Rifle is not tied to the Rangers in any way. Likewise the Desert Ranger armor in Zion was from a Marine serving in two cities on China's east coast, not the Gobi. Having joined the NCR they function in pretty much the same capacity, they are just no longer autonomous.