On Tsavo Highway have somebody boost at the turret part of the warthog while you are driving (you must not move and have to have full health)

This happened to me by accident while I was doing campaign on legendary.

For the chopper you must damage it some, but don't destroy it.

Then hijack it from a brute from the driver part, if done right you will be "driving" a destroyed chopper (both happened to me when you face the 2 or 3 wraiths and choppers near the end of the level).

If you get out once it happened you can't get back in.