MOST people
NEVERchange gear oil in
the Diffs, let alone the Trans or T-case. As Zuke lovers
we tend to take better care of our vehicles, tho 9K miles
is more frequent than most people would do, if you think
about the offroad conditions, water crossings, mud, and
dust in general, this is a good idea.
I would say once a year should be enough, but oil is fairly
cheap insurance, kind of like the guy who just spent $1000
rebuilding a nice trick engine, and doesn't want to dump
the oil after 50-100 miles, even tho it only cost $5

The fill amount is to the plug in the rear of the diff, take it
out, fill until it starts to come out, done.
I put Slick50 in the diffs when they were new, and when I did
the rear locker I switched to Synthetic oil in the rear.
I hear racers like to run 30 Weight for less friction, tho they
get new gears every race too. Would work in a pinch, better
that nothing, but not for long term use.