There are a few different reasons that can cause rear differential failure. One of the leading reasons is dirt and debris entering the differential. This can happen if the vehicle isn’t properly serviced and the differential fluid isn’t changed regularly. Another common cause is manufacturing defects. In some cases, the differential might not have been properly built, which can cause it to fail over time. This is usually covered under a manufacturer’s warranty, so if you think your rear differential has been unable due to a manufacturing defect, you should contact the manufacturer to see if they will cover the cost of repairs.