I discovered an identical leak on my transfer case today. I'm at 29,000 miles on a 2016 SR5. I'm headed to the dealership tomorrow for (hopefully) a fix under warranty.
I have done only light offroading and have done no mods, so this is a huge black eye on Toyota's supposed reputation for reliability. If I have to have major drivetrain repairs every 30K miles, I might as well just buy a Jeep Wrangler next time around.
UPDATE 1: Toyota dealer didn't really fix. They added dye to the fluid to track the leak, and asked me to return in 500 more miles. We'll see what happens then I guess -- will update this thread then.
UPDATE 2: Second trip to dealership this weekend. Was still leaking from rear power output of transfer case, very slight leak. They had to order a new seal, and they're going to replace it. Taking them 3 days roughly for the repair. The reps at the dealership were incredulous -- could not believe I didn't wreck this myself somehow, or that I bought it new and already have a leaking tranny. Me too haha!
UPDATE 3: Truck has now been at the dealership (Toyota Marin) for a week. They ordered a new seal, pulled the transfer case apart, and then the new seal was no good for some reason. Tech is supposedly ordering a new seal to be delivered tomorrow, at which point they'll reassemble and be done with it. After reading the stories above about problems with casting on the transfer case itself, I'm not convinced this will actually solve the problem. Dealership says no service bulletin is out on this issue yet, but watch for more in coming months and years. The way the cars are built using highly controlled robotic processes, if there is 5 of these out there, then there are probably 1000 out there waiting to be discovered.