The rear differential swap or just the center section will be lots less work and cheaper if you can find a ratio you like. On the other hand an auxiliary can be more useful because of the extra gears... but even 0.73 OD won't do but so much to overcome exceptionally low axle gears. All depends on what you have now and what you want to do in the end or what you can live with as far as speeds and RPM.
Grigg
