In Nikon DSLRs is the autofocus mechanism in the body or in the lens?

The focusing ring would technically be the mechanism, but the motor that turns it could be in the lens, body or both. To make this more confusing, Nikon DSLR's have what's called a "rangefinder" function that will let you know when the camera thinks that your selected focus point is in sharp focus. This works on all lenses, either manual or AF.

Both body and lens (if available in the lens)

Most lenses have motors, some do not.

Most bodies have motors, some do not.

If you think this makes buying a Nikon much more complicated than it aught to be, you'd be correct.

Depends on the lens / body . Sometimes the motor is in the lens , sometimes in the body , other times both .

In most modern lenses, but not in modern lower market bodies, nor in older lenses.

Yes! In both! Some camera bodies have AF motors and some lenses have AF motors.

Body mounted AF motors are in the high end bodies.

Some, it depends, either, both! Confusing?

There are Nikon bodies with autofocus motors and there are bodies without. Then, there are Nikon lenses with autofocus motors and there are lenses without.

Low-end Nikon dSLRs like the D60, D40, D3x00, and D5x00 models don't have an internal autofocus motor. To autofocus with these cameras, you will need to use lenses with their own internal focus motors. With Nikon, these are marked by the AF-S, AF-I and AF-P specs. AF only is not enough.

Mid-level to high-end Nikon dSLRs have built-in autofocus motors so they will autofocus with any Nikon mount AF marked lens whether the lens has a built-in motor or not.

Both. There is a sensor in the body, in the plane of focus, that determines whether the lens is in focus or not. It controls a little motor in the lens that does the focusing. This is why for autofocus to work you need both an autofocus body and an autofocus lens.