Where did you find those recommendations? I found info about my Dana 80 to have the oil changed every 30k.
I rebuild my differential a year ago and changed the oil after 10k as the plug magnet had lot of steel "dust" on it.
That said my Mercedes has 220,000 miles on it and I have no records the differential was ever serviced. I am not planning to do so for the next 300,000. Only checked the oil level some time ago as it shows some oil sweating.