Mi Grand Vitara 2001 XL5, breaks the municions

Hello greetings to all friends.

Does anyone know the reasons why my Grand Vitara XL5 synchronous truck breaks the rear municists? I have it with me since July 2008 and on each rear axle I have had to change at least 5 municases with steadras on each side. Again broke the axle tip. I thought if the transmission pine nuts are spent or mismatch influence the life of the municasers?