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U.S. Gear took over the manufacture of the Doug Nash "Dual Range". The manuals are identical from either one, only the name changes. Apparently, U.S. Gear changed the pressure angle of the gear teeth and increased the number of teeth on the planet gears to address an apparent noise/backlash complaint. Also, they uprated the G.V.W. of the unit due to these changes. Some parts are not interchangable, that's what U.S. Gear told me anyways.
The shift motor will work from a two speed rear end. Also the shift motor from Nash is wired differently than the U.S. Gear unit. One grounds the motor and switches power between the two terminals, the other switches positive and negative between the two terminals. I can not remember who did what however.