mechanic said the Posi traction in differential was worn out is this common ?

What is your question?

Needed the differential oil changed, the mechanic said the pois track was worn out he showed me the difference. also said the axles tend to be rounded typical of these trucks. I want to know if the axles wear out that fast. He suggested good used parts for the differential, What would a typical repair like this cost,..

What makes this problem better or worse?

Old oil made it worse, feels like new oil in differential gives my truck better traction

How long has your 2002 Mazda B4000 had this problem?

2 yrs