Originally Posted by Jetboy

The early sign is that gear lube will being to leak down the inside of the rear tire. Generally it would be going through the drum brake on older models.

What it means is that the axle gear lube has passed through the bearing seals. The rear bearings are sealed, so once gear lube has passed through the rear wheel bearing, the bearing is shot. You could potentially seal the rear axle to keep the gear lube inside the axle, but that would not solve the problem that the rear wheel bearing is no longer lubricated by bearing grease as it should be. The result will eventually be that the rear tire would fall off. When it gets bad - you can lift the rear axle and feel that the rear wheel is loose.