This is one of the six videos I have posted concerning the 2009 Moab Ural gathering. Many times in these videos I will purposely go slow when going up or down rough stuff to give a much better idea of how nasty some of this stuff is, this happens in the first part of this video.
Fast forward to about 4:45 in the video you will see one of the rigs is in ONE wheel drive and slows down because of the rough roads and rocks, he has a hell of a time getting going again and is passed by a couple of other rigs who are in two wheel drive, you can tell they are in two wheel drive as watch close and you can see both the pusher and the sidecar wheel kick dirt to get and keep moving. I forget the riders name but you can see once he shifts into two wheel drive he has no problem getting going again.
Like has been said and I agree, two wheel drive is not needed but it sure comes in handy even when doing semi-sane rough road riding as this video proves. It was about a three mile climb from the bottom to the top of these switch-backs, its on such terrain that its a heck of a lot easier on the rig to just put it in two wheel drive at the bottom and leave it there till you've reached the top.