While you can become a Ranger in Fallout 2, you can't join them in Fallout: New Vegas because the NCR Rangers aren't an actual faction. However, you can get pretty darn close, at least in aesthetic. Complete quests for the NCR to raise your reputation with them to at least "Liked." After doing this, speak to Colonel James Hsu, who will give you the key to a safehouse located just south of REPCONN Headquarters. Inside the safehouse, you'll find all kinds of NCR goodies including the iconic suit of NCR Ranger combat armor and a ranger helmet. This armor is the same kind that's worn by veteran NCR Rangers and is featured on the cover of Fallout: New Vegas.