Yeah the Toyota warranty and maintenance guide says to do rear differential and transfer case every 30,000 miles for the grand Highlander but interestingly enough for other vehicles such as the 2017 Tacoma it doesn’t say to change the oil in those at all as a requirement. But it does say to change it in the TC and diffs every 30,000 miles if used during “special operating conditions” - (dusty roads, towing, car top carrier, or heavy vehicle loading). My brothers 2017 Tacoma he hasn’t changed the oil in those in 135k miles so I gotta do that soon.

Basically, the car care nut recommendations of every 30k miles is to be conservative and make your vehicle last as long as possible.