4WD models have an additional sensor in front of the radiator that measures deceleration. I'm not sure why 2WD's don't have it and 4WD's do, but I imagine it has something to do with 4WD's having a higher likelihood of having an offroad accident and perhaps it adds some extra sensitivity?

In any case, I know ARB makes all their bumpers airbag compatible and since all the other ones for the Xterra are pretty similar I wouldn't expect much difference. The bumper mounts to the accordian section of the frame and doesn't affect the crumple zones.

And with all this, remember that seatbelts save more lives than airbags do. The airbag is an "extra" restraint that is intended to supplement your seatbelts. Always wear your belt and you won't have to worry so much about the airbag.

And remember, front airbags only work in severe frontal impacts.