Unlock All Vehicles/Level Select:
To unlock all of the vehicles, go to the player select screen and press Y, Z, X, L.

Unlock Demolition Derby Mode:
To unlock, win a match without firing weapons.

Unlock Kevin Nash:
To unlock Kevin Nash, go to the character select screen and press L, X, Z, Y.

Unlock New Drivers:
To unlock the following drivers, complete the tasks listed:
Bradshaw: Beat Season Mode with Brock Lesnar.
Christian: Beat Season Mode with Chris Jericho.
D-Von Dudley: Beat Season Mode with Bubba Ray Dudley.
Hulk Hogan: Beat Season Mode with The Rock.
Lita: Beat Season Mode with Matt Hardy.
Ric Flair: Beat Season Mode with The Big Show, Rob Van Dam and Triple H.
Stephanie McMahon: Beat Season Mode with Kurt Angle.
Vince McMahon: Beat Season Mode, facing Vince in the final battle.