After a few thousand really sharp pics, all of a sudden the shots are coming out slightly fuzzy, hardly noticeable at normal resolution, but especially evident when enlarged. Tried several different lenses with no difference. Doesn't seem to be related to settings, as we tried several with the same result. Same difference whether raw or jpeg.

If the AF is set to release then the camera will release the shutter before focus is attained. Set it to focus only and see if it improves. Don't leave it like that, you will miss pictures.
Shoot some pictures with the center AF point only in single release and see if the focus is consistently landing in front of or behind what you wanted in focus. A penny on the sidewalk shot at a 45 degree angle in aperture priority with the lowest A number possible is a good quick one. For super fine tuning with a f2.8 or faster use a fuzzy plant branch like a tomato plant. If whats in focus is always in front of or behind what you focused on then you need too adjust the focus(back focus) in AF fine tune until it lands where it should. It's an annoying process that involves shooting, adjusting, shoot again, see if its better or worse, adjust, shoot and repeat. On zoom lenses use the middle on the zoom range because it will usually go from back to front focusing on the long and short end. The camera remembers each lens and the process must be repeated for each lens.
Last thing I can think of is the one that no one wants to hear and is most likely the problem, Camera Handling. People have a tendency to get sloppy after getting use to a camera. I have noticed that the VR lenses don't like people that stab the shutter button. It wants to remove fine vibration and a harsh stab makes it jump back to center and if the shutter opens at the same time it will put fine motion blur in the picture and looks out of focus.