Forgive me if this sounds totally stupid.. but isn't the purpose of having a two speed rear end so that you can get higher highway speeds?
If thats the case, couldn't you just find a two speed rear end out of a simular year vehicle and just swap out the rear axle?
Theres a dude with a yard down the road from me who has several 50's 1.5 and 2 ton trucks, and has a few GMC axles (both vac 2 speed and "regular") already chopped out.. don't know if those would do you any good, or if getting one from western Nebraska would even be cost effective, but you could probably get 'em cheap.
