I'm not sure we are sure if crabby has specified what kind of twin screw he has. The setup 3B describes was a short lived option on medium duty Chev for a few years. Rockwell already supplied an inter axle differential at that that worked similar to modern twin screws. The axle with the least traction gets the power. A power divider lock locks this interaxle, but cannot be used on hard surface roads. It is only to be used in emergencies. Lockers are also available that lock the individual differentials. If he will post a picture or a model number, I can tell him how it works.