I had thought about this at one point. To get over the center differential and drive train binding issues, my idea was to include a v-belt, pulleys, and tensioner as a makeshift "transfer case."
Hook the tensioner to a hand lever and that will let you select between RWD only or 4x4.
Also, since the V-belt can act as a clutch by varying the tension, you could set the tension just slightly loose...just enough so it slips when the drivetrain wants to bind but grabs when the rear tires what to break loose.
You would basically have a pulley on the rear axle, a belt between that and another pulley on a jack shaft. The other side of the jackshaft has a sprocket, a chain between that and another sprocket on the front axle.