A slight rear differential leak or a slight bearing whine is sometimes tempting to ignore.  A proper repair involves disassembly of the differential and replacement of the worn parts.  Doing nothing is far more expensive.

There are many causes of differential problems.  Water sometimes dilutes the lubricant.  Other times the oil is not replaced and over time breaks down.  Still other times a bearing will fail and cause a seal to leak and/or gear misalignment.

Rear differential gears operate at extreme pressure.  A misalignment of a few thousandths of an inch can destroy them.  When the above pinion bearing failed, it was due to a breakdown of the lubricant.  The oil was not replaced for over 150,000 miles.  

Replacement of the bearing and oil may have bought several additional years of service.  Instead the vehicle was driven a few months with the bearing noise.  Eventually the load became too great and the ring gear failed.

The broken teeth jammed, tore the rear cover and locked the rear differential.  In that instant, a few hundred dollar repair became a few thousand dollar repair and a wrecker bill.

Rear differential leaks and noises can normally be repaired if address immediately.  Better still they can often be prevented with maintenance.  Doing nothing is a VERY expensive option . . .