Why not use a Dana 60 rear axle flipped upside-down and facing rearward? Just don't forget to weld the vent shut and drill a new one. Another thought, although I'm not sure how well it would work, is a 4X4 transfer case, just running through the "front" shaft. Thinkin' maybe the old iron New Process from a 70's Dodge pickup. Think the number was NP203. This would give you 2 speeds, "4 HI" and "4 LO", which should both equal the front 2 High!
