We tow a boat, so I change the F&R diffs every 15k and the XC every 30k. Per owner's manual. Because of the frequent changes, can't justify use of synthetics, so just use the vanilla dino stuff.
It depends if you tow or not. No towing with synthetics should be an easy 60k to 100k.
Towing then becomes "it depends". Heat and pressure will break down any oil, so pulling at limit, uphill in heat and a 30k interval could be warranted.
Me? About 100k but I use the Shell Spirax S (truckers use it and supposedly a 1,000,000 mile interval or something)