This adds options for all the stock rear end ratios from the hopper, but reversed! Simply install the final drive ratio you'd like that has the word "reverse" in it, and voilà! You should be able to drive fine in 2WD (opposite direction of transmission) but spin in place (mostly) in 4WD. Try to experiment with lockers, high/low range, and different surface types.
Inspired by this Garage54 video: