Tricks:

Get Monster Truck:
Go to dinosaur level and drive back wards for a long time.

Get Monster van:
Go to swamp level and on the keyboard hit up and left or right keys. Be careful landing!

Get Green Van:
You'll need a joystick to do this. Go to the Garden level and on the joystick go side ways hitting the 3 and 4 buttons. Listen for a gun shot and try to do 4 of these.

Get Tydied Van:
Do the same thing that you did to get Green Van except you go to the beach level.