They are drivable, me and my friend got them inside and were able to drive them. The key that many forget is that you have to blow up the carrier warthog, wait for it to respawn, and then get it over. If you don't blow it up and bring it inside, you won't be able to drive it.
Yeah, you use grav lifts. You can't bring it over the fence, so you have to put it over the far wall to the right, drop it, and go below the map to push it back over the wall, but on the other side of the invisible wall, so that it lets you push it over.