To get this you have to get first place in season 4 of career mode in the power division.
Get Moon Buggy
To get a new level and the moon buggy, destroy the signs that are black with a white circle and make a bunch of dots on the screen.
Helicopters and hang gliders
Helicopters and hang gliders will fly right through you and cannot be destroyed. If you hit one while in the air, you will go through it.
Humvee
To get this you have to get first place in the first 3 seasons of career mode in all divisions.
Infinite Class Trucks
To get this you have to complete the first 27 tracks in single race mode.
Moab: Drive on arches
Drive around until you see a large arch and try to drive up it. You need a power vehicle, such as the Moon Buggy, the Hummers, or the Monster Truck to do this.
Moab: Go over large hills easily
When using the freeroam option, use the modified Jeep Wrangler to go over large hills.
Moab: Knock down rocks
When first starting out, go straight until the first larger hill. Go to the left and look for a rock structure that comes to a point and a big ball of rock at the top. Drive up the low angled side and go directly into the rock. It should fall down into the crack to the left side. This cannot be done in multi-player mode.
Monster Truck
To get this you have to complete all 36 tracks on the hard difficulty setting in single race mode.
Pro Class Trucks
To get this you have to complete the first 9 tracks in single race mode.
Rod Hall Hummer
To get this you have to get in first place in all divisions in career mode.
Shelby Dodge Durango
To get this you have to get first place in season 4 of career mode in the speed division.