The Search Begins Again...
I've run new tests...very regimented...tests between the lens that came with the D80 and with an old manual Micro-NIKKOR 55mm 1:2.3 that I used 20 years ago with my Nikon FG. I knew the 55mm was an excellent lens. I had the camera on a tripod, took a photo of a small heather plant I have with the 55mm, then changed to the 18-135mm autofocus lens that came with the camera. I set it for 55mm and let it self-focus. I centered my focus on a particular branchlet for each.
The top photo below is with the manual 55mm, the lower photo is from the 18-135mm. I carefully cropped both to look the same so the eye would not be distracted by different branches showing in the image. Neither are sharpened, both are shown at full resolution (100% size). The camera was 6-7 feet from the plant, so this photo is cropped out of the center few inches of a 53" wide photo.

I then packed up the D80 and returned it. (The store had already told me they would not swap out the lens for another.) At least now I know I'm not nuts, nor am I asking too much of a lens.
Again, thank you all for your help. For now I'm back with my CoolPix 7600. I'm restarting my research.