when should I change my transfer case and differential oil in Toyota Sequoia 2008 5.7L 4WD?

Yes, somewhere around 100,000 miles is a good time to change transmission fluid and differential gear oil if you plan on keeping a vehicle indefinitely. But it is not that critical and some people never do it. I bought a like new 2008 Subaru Outback with 150,000 on it 1.5 years ago and the dealer had just replaced all belts (incl timing belt) and fluids (including differentials) and brake pads. So I expect to get many more years out of it.

One thing to be aware of is that if either or both of the differentials have limited slip, they need a friction modifier. With my 1986 Ford F150 that was a separate additive. But when I did not see that on the invoice for my 2001 Pathfinder when I had differential fluid changed, they told me that they used gear oil that already contained the friction modifier. With limited slip rear it got around in deep snow in just 2WD as well as a GMC pickup in 4WD without limited slip and in 4WD the Pathfinder with limited slip rear was definitely better (as was my 86 F150).

every 30,000 miles

Just change them. As far as I'm concerned, they are overdue.

ADDED: I have a limited slip differential and my repair manual says to use synthetic 75W-90, no additives required. You are better off replacing your oil now and not worrying about any additives.