After several months I realize the only truly sharp D800 >images I have were done with a tripod
A good indication of root cause right there. I have noticed that I need to take exta care bracing the D800 and holding it still when shooting handheld. I am also typically using about 1 stop faster shutter speed than with D200/D300 that I had previously.
I have used every focus mode but have relied most on AF-C >d51, manually moving the focus point, expecting that to >provide the best results. All I can say is: It is doesn't. >
Inappropriate; unless shooting moving subjects that move fast across the whole frame I would try something else. AF-S for static subjects. The avian shooters seem to prefer AF-C d21 for birds in flight.
There are a host of custom settings that alter responsiveness. If we knew what type of photographs you mostly do, and the applicable lighting conditions, could offer some more targeted advice. My D800 required a few changes from default settings before I was happy with it.