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