As mentioned in an earlier post, my bus also has the "locking differential" feature. I can electronically actuate it from the dash. I believe this would be used if you are on a soft, sandy or muddy surface.
I have not had my bus buried in the mud yet and I will continue to avoid it, but I am from time to time in need of extra traction on ice.
I will usually lift the tag axle to allow all of the weight transfer to the drives. Once I get moving I drop the tag back down and your on your way.
It Works !