Yes, I know what you are saying. In my attempt to explain my point, I oversimplified (and used some "technically" incorrect terms, admittedly).What you're calling a "center differential" is known as a "power divider". Yes, it does the same job front to back as a differential does side to side, just a different name to help keep them separate in our minds I guess. Not many 4WD vehicles have power dividers. They don' t have "locked power dividers" as you suggest, they simply aren't there. Front and rear drive line are both turned the same, all the time, when 4WD is selected. This is called "full time 4 wheel drive" as opposed to "All Wheel Drive". That's why you'll always get one front and one rear wheel spinning (or three wheels spinning) but never only one wheel spinning.
You can put whatever size wheels and tires on a 4WD vehicle you care to if you engineer the difference into the transfer case.
So you are saying that tractors have tire size difference ratio accounted for by gears in their power divider? And it one wanted to change tire sizes, they'd need to also correct these gears, correct?