hey scotty , i have 2011 Xterra with 210K KM today when i wanted to change the Rear Diff oil small parts came out and told me its better to return the old fluid back to the diff , so should i change the fluid like 1 liter ever 5K so it would be clean over time or should i leave it till it break , or buy rear diff from junkyard , or just buy upgrade from ARB witch its expensive so i can use it better , keep in mind im only using the xterra to go to the desert only i have other cars for daily use , every thing good with the car front diff clean oil , tranny , engine i use german oil for 10K KM and i change it at 2500KM ,so early to it last as long as possible.
why did you put junk back in the diff? 
i did remove the contaminants , cuz he told me the rear diff could go out
what kind of contaminants? Did you take a picture?
are you sure he didn't pull those pieces out of his pocket?
I would just find a junkyard axle.
yeah im sure i did look inside the axle from filling point and i did find broken peices there , i just go to these mechanic just to change oil cuz they are cheap i whould never trust them to leave my car 1 min without my eyes on him