I would try it on a firm dry surface and see how hard it is to get it into and out of 4x4. That will tell you how bad/good the tires match up. You can also lower the air pressure in your rear tires, this will help bring the ratio back to where it was with the smaller tires.
On a bit of a side...
With my 1100 C, the cyl is connected to the box by a pin, I think I would try removing the R.R. wheel and removing the pin. You will need to be careful as once you remove the pin, hose or whatever, the box will only be controlled by gravity.
No 4x4 dash light on the grand L either, rather cheap of kubota if you ask me. There is also a recess in the knob of the lever for a emblem that they could of used, instead of a cheap little sticker stuck to the body of the cab.