The center diff lock is the difference between true 4 wheel drive and "full-time"/all-wheel drive. If you're driving on dry roads with lots of traction, you should have the center diff lock unlocked. This allows a difference in speed between the front and rear wheels so you can corner normally on dry roads. When you lock the center diff, it is exactly like the old-style true 4 wheel drive where the transfer case drives both the front wheels and the rear wheels to turn at the same speed. The front left and the front right tires can still vary in speed, but the average speed of the front wheels is forced to equal the average speed of the rear wheels. So locking the center diff is ideal for things like snow and rugged off-roading. Locking the center diff is bad for driving on dry pavement because it doesn't allow different speeds between front and rear wheels, which is needed to corner properly without squeeling tires or worsening handling.

You have some other good questions there that I don't know the answers to (like ratios, etc). I know the Sequoia shares front suspension components with the Tundra, but I'm not familiar with the rest of the drivetrain.

I don't have VSC or A-TRAC, so someone else will have to tell you about those. Those started in 2001.