What you need to do is have the guy jump outof the back of the truck so that he is standing right behind it. Shoot him so that all his granades fall to the ground. Then, jusmp back into the car, and park over the granades. get out of the car once more, and toss a granade of your own under the truck (does not have to be under, just close.) Boom! car goes flying. the more garandes under the warthog the higher it goes. Oh, it is best if the truck is parked aginst the rock... that way there is more force blowing the truck skyward.
not sure of the level name... but it is the only level like that so I am sure that you are thinking of the right one.