These are all of the trucks that you can unlock in the game. There is one truck that is missing, you must play multiplayer with a full group of people.

Truck	How to unlock
Avenger	Earn 4,000,000 Moster Point
Backwards Bob	Win the Speedster Series
Black Stallion	Max out your Obstacle Usage during a Freestyle session
Blue Thunder	Earn 1,000,000 Moster Point
Bounty Hunter	Earn 2,000,000 Moster Point
Bulldozer	Earn 2,750,000 Moster Point
Captain's Curse	Fly over 160 meters in Monster Jump
Destroyer	Earn 1,500,000 Moster Point
Devistator	Win the Smasher Series
El Toro Loco	Gain 5 Monster Spectacle Points while racing in London Rails
Grave Digger (Original)	Win the World Series
Iron Outlaw	Win Mall Madness in less then 2 minutes and 30 seconds
Jurassic Attack	Knock a wheel off while racing
King Krunch	Earn 3,500,000 Moster Point
Monster Mutt Dalmation	Score 30,000+ Skill Jump Points
Spike Unleashed	Earn 4,500,000 Moster Point
Spitfire	Perform a double mid-air combo
SUZUKI	Win Sam Boyd Stadium Racing in less then 10 seconds
U.S. Airforce Afterburner	Win the Stuntman Series
War Wizard	Lead every lap of an Eliminator Race