To unlock the eight monster trucks you need to do the Champ Tour. The eight monster trucks you can unlock are Grave Digger, Maximum Destruction, Avenger, Redtoid, El Toro Loco, Inferno, Predator, and Eradicator.
Race in every offroad and collect all 50 stars to unlock the Moon track. Go to the training mode and race on the Moon. On this track you can collect 27 stars. Each one represents a man or woman that worked on the game.
Offroad Challenge Stars
Use training to get the stars in the offroad challenge. The Star Sensor helps you find the stars in the offroad challenge.
Unlocking Trucks Easily
Set difficulty to easy and complete the World Tour. You will still unlock every truck. Under harder difficulty settings, if you do not win or finish well in an event, you will not unlock the truck. If you do not win once you get to Nevada will not unlock Grave Digger.
An easy way to collect all fifty stars is to race in the offroads in the race event or training mode. You can still collect stars in both modes. This makes it easier to collect all fifty stars.
During the course of an offroad race, do a somersault. You will collect $500 for each one.
Watch for living creatures during the races. For example, in Louisiana there are alligators in the swamps, in southern California there are dolphins, in Nevada and the Moon there are aliens. There is also an alien ship in Nevada.