I have two auto tranny Chevys out front that are 30 and 32 years old
respectively and I'd bet they've never had their differential oil checked,
never mind replaced. I'm not proud of it, it just never made it on the list
(for a long time, due to health reasons and such, something had to be
actively breaking or broken to get on the list, but that's another story). I
did change my diff oil when I rebuilt my Vgon tranny and it was darker than
the new stuff but not terribly so. It was still plenty gooshy, too.

I tried the funnel-and-tubing bit and since I was concerned about missing a
couple of birthdays out there in the carport waiting for that crap to make
its way down the tube I went with the quart bottle with the pointy cap,
which got it in there pretty quick. I found that I could lie under the van
(with it on its tires) pretty easily and I had plenty of room to raise the
end of the bottle and get the stuff in. No muss no fuss. The worst part was
making a "gasket" out of "The Right Stuff" and getting the pan on there
without screwing it up.
I found a fairly inexpensive 3-piece set of metric hex sockets with a 17mm
and 3/4 drive at Autozone, btw.

Cya,
Robert