I have heard of people doing that, however I personally wouldn't go THAT low unless ALL of my miles were hwy unloaded with smaller hwy tires on a 2wd.
Do you have a 2 or 4wd? If it's 4wd and you only need to replace the rear gears then you'd be best off sticking with stock gears so that you don't spend boooo-ku-bucks to change something you don't really need to.