I think rearend gear lube is supposed to be changed periodically just like both type tranny fluids as well as brake fluid and coolant just to protect everything like it should be.
I do know the rearend gear lube in the old P/U I bought a while back was what I would call, NASTY"!! I flushed it out as good as I could with spray Brake Clean and wiped as much as i could out of it and filled it back up with new 85w140 gear lube.
I'm going to drive it a while and change it out again after I get the rearend warm from driving it. Flush her out and fill her up again sorta deal....
I see it as being one heck of a lot cheaper maintaining it then rebuilding it since I don't know how to set one up right AND would have to pay to have it done right??
pdq67