Complete the ATV Academy and Trick Training objectives to unlock the Dimension and Viper ATVs. Win eight racers or more in Arcade to unlock the ATC ATV.
I decided to condense the cheats given into one large section so they are easier to read.
Create a new profile and give any of these as a name:
All Riders: BUBBA
All Tracks: ROADKILL
All Vehicles: GENERALLEE
Get Challenges: DOUBLEBARREL
Get Champs: REDROOSTER
Max Out Skill Level: FIDDLERSELBOW
Max Out Stats: GINGHAM
To unlock, Get Challenges - DOUBLEBARREL
To unlock, Get Champs - REDROOSTER
To unlock, Max Out Skill Level - FIDDLERSELBOW
To unlock, Max Out Stats - GINGHAM
type your profile name in as goldrush to unlock every thing