My research has led to a website
http://chevy.oldcarmanualproject.com/index.htm
which has lots of old manuals on it.
I have attached illustrations showing exploded diagrams and parts lists of a 2 speed differential, a standard axle for comparison and diagram of the linkage to the cab although it doesn't show the cable connector to the speedo adapter.
Its not clear to me whether the 2 speed unit bolted on to replace the conventional axle differential casing at the front of the axle, or it came as a complete replacement axle. I haven't yet found a good clear photographs of either axle for comparison.
The front axle was 100mm wider than the normal axle to allow for the extended fenders and sand tyres. I presume the rear axle would also be extended. Would this be a new axle, or would they just have extensions to the standard axle?