I actually have no idea what axle was used in the Napco conversion. Given the challenges of the other approaches, I'd want to find out before committing to an overdrive in this case.
I've spent some time thinking about overdrive on a four-wheel-drive and aside from an OD transmission I don't like any of the choices, and without moving the transfer case it may be very difficult to convert this truck.
But I'll admit I know very little about the Napco stuf...
