You never 'need' a monster truck in this game.
The only place I found a Monster Truck was at Mike Toreno's house after I did his racing mission, I just put it in a garage.
South Park, Charmed, Buffy The Vampire Slayer / Angel, Tru Calling, Family Guy, TrippingTheRift, FreeForAll, Futurama, Jackass, Lost, That 70's Show, Celebrity Deathmatch