Unlock El Cajon Track
At the Level Select menu, press L + R + Up on the D-Pad. If done correctly, you will hear a drill sound. Then, highlight El Paso and hold Z + A.
(Thanks to Davvid_9889@webtv.net)

Unlock Flagstaff Track
At the Level Select menu, hold L, then press Left on the D-Pad. If done correctly, you will hear a drill sound. Then, highlight Mojave and hold Z + A.
(Thanks to Davvid_9889@webtv.net)

Unlock Guadalupe Track
At the Level Select menu, press R + Down on the D-Pad. If done correctly, you will hear a drill sound. Then highlight Vegas and hold Z + A.
(Thanks to Davvid_9889@webtv.net)

Unlock Monster Trucks
To get monster trucks, press the corresponding button once at the car select screen, then press again to show the default truck.
4x4 Monster: Up-C
Punisher: Down-C
The Crusher: Right-C
Thunderbolt: Left-C
(Thanks to Davvid_9889@webtv.net)