If you have the old tires, or a tractor with them on it, measure the circumference of the fronts and rears, and calculate a ratio. Then do the same with the new tires. It is possible that both front and rear tire sizes changed slightly.
Most vehicles should not be driven with the front differential locked on pavement or surfaces that don't slip a little.
Do any tractors have all the time 4x4? If they do, then they would need some kind of differental connection or hydraulic system allowing the power to be different between the front and back.
I think I've heard that with cars it is preferable that the fronts pull slightly more than the rears, or at least no less than the rears as it is better for steering. I.E. too big of front tires would be more forgiving than too small of front tires.
My guess is there is a little leeway with tractors.
Consider a tractor that has 60" rears, 30" fronts. Ratio 2:1. Grind off 2" from both the front and back, 58:28 gives a ratio of 2.07:1. New rears, bald fronts, and one gets 60:28, or 2.14:1, and new fronts and bald rears, 58:30 or 1.93:1