Thank you both for your quick and helpful responses. I used the Vignetting tool, but to get it to remove the slight dark corners, I had to select Manual as Auto did not fix anything, and did not do anything as best as I could determine even when I moved the slider. In Manual, I had to get the slider almost all the way to the right to fix the problem.
The Kenko tube used was a 35mm, and one designed to be used on Nikon mounts. I did not test out the impact of different apertures, but I am pretty sure that the vignetting effect is greater the smaller the size of the actual aperture.–f 16 produces moire vignetting than f 5.6.
To avoid vignetting with Nikon lenses, it is best to use Nikon made extension tubes. When they are used on “most” Nikon bodies, information is transferred, but AF is not possible.