Is there a vignetting at the widest aperture of the 18-200mm f3.505.6 VR Nikkor lens? Whenever I took images at the 18mm I noticed dark areas in the corners.
If you are using a lens hood is it properly oriented? If you are using a filter, is it a thin filter? The filter can make a lot of difference at wide angles. These 2 things are what comes to mind first. There may be another problem but I don't have enough info for any other guesses.
I am using an 18 to 70 on a d200, and I have no problems with the lens. I agree either you have a problem with the hood or there is something mechanical going on. I realize it is a different lens, but same focal length by Nikon, and a long way from extreme wide angle.