A couple of conditions that cause premature pinion gear failure.
A lack of lubrication and incorrect lash. Incorrect lash of gears that are fixed in manufacturer ( non adjustable ) is often caused by bushing wear. Even with lubrication too much lash puts undue stress on the gears and it's usually the pinion of the two mating gears that fails first.
Well, the wheels should have new bearings. I would changewould you please explain lash??