There is no cheat code to spawn a specific vehicle in GTA IV on Xbox 360. The in-game phone cheats only alter gameplay states like health, weapons, and weather—they don't spawn vehicles.
Note: The Landstalker is based on the 2003–2006 Ford Expedition/Lincoln Navigator, not the Chevrolet Tahoe (which resembles the Cavalcade). The Cavalcade spawns in Algonquin/Star Junction.