Gear lube in Brakes!!!

You may want to google articles related to the 'short fiber grease' that is referred to in the Dodge FSM's for rear axle bearing assembly; it apparently is not just any wheel bearing grease, or what you would put into a grease gun. It is a certain sodium soap based grease.

Why does it last? Well some small amounts of gear oil WILL get past the inner seal from time to time. I suspect it keeps the whole area around the bearings well lubed and supplements the initial grease load. Take apart an old 8-3/4" axle and see how much grease is still in there.