When I got home after shooting over 400 images, I found most were soft... almost as if the AF had failed slightly. Obviously, I wasn't happy.
I have many sharp images from this camera/lens combo so it seemed unlikely to be a gear problem.
To figure this out, I made some test shots in the yard just to find them all nicely sharp, as usual. I tried several AF modes and variations to rule out gear issues, but didn't see any problems. After all the re-testing, I realized the only difference was shooting from the car.
So I took the car out to a local park and photographed some trees (which move at about the same speed as the Bison I photographed in the refuge) while the car was running. I paid more attention to what I was doing. I realized that my natural position shooting out the driver's side was to rest my elbow on the armrest. And I noticed vibration in the viewfinder when I did so. The shots were just a bit soft... so obviously, even at 1/1000s, the car vibration impacts the image sharpness.
In retrospect I think this problem should have been more obvious to me. I had a similar problem shooting from a tripod on a busy boardwalk; that was the point I started hand-holding for wildlife. A tripod or any other support becomes a liability on shaky ground.
