There are a lot of drivetrain permutations:
In all 2WD models, the 4Runner has an "Auto LSD" which uses the brakes to simulate a rear limited slip differential.
In 4WD Limited and SR5 models, the 4Runner has open diffs front and rear but includes A-TRAC which simulates front and rear lockers by braking selectively. This is surprisingly effective.
In TRD models (4WD only) the 4Runner has a physical locker on the rear diff and also A-TRAC to simulate a locker on the front diff.
In 4WD SR5 and TRD models, the center diff (
EDIT: it's not a differential on these models, just a transfer case) is either disengaged (2WD mode) or locked (4x4 mode). These are part-time 4x4 systems that must only be engaged on loose surfaces (dirt, sand, etc).
In 4WD Limited models, the center diff is a locking Torsen which is full-time AWD (basically a limited slip center diff) AND can be locked into the same 4x4 mode that the other 4WD models can be.
If you want a detailed breakdown of exactly what hardware comes with each trim level, I suggest checking out the
2017 4Runner PDF brochure (scroll to the bottom for a feature comparison matrix).