Lens Reviews tested this lens and found some interesting results, see here.

http://www.imaging-resource.com/lenses/nikon/18-105mm-f3.5-5.6g-ed-vr-dx-af-s-nikkor/review/

The main point is near the bottom,

Shading (''Vignetting'')
The 18-105mm does show some evidence of corner shading, but its presence is fairly subtle. If you demand absolutely no corner shading in images produced by this lens, you'll have to use it at apertures of ƒ/11 or smaller; larger apertures result in some form of corner shading. Using a focal length of 35-50mm will help; these focal lengths are a bit more forgiving, and will only produce significant corner shading when the lens is used at apertures wider than ƒ/8.

At the focal length extremes, upwards of 2/3 of a stop of corner shading can be seen when the lens is used at its largest aperture.

and also here

If there's a weakness in the lens at all, it would be performance at 105mm, where image sharpness doesn't quite reach the level seen at other focal lengths. However you'd be hard-pressed to see the quality difference in practical use.

The lens received high praise over all from the reviewer, but did state the same issues you discussed.