An AWD car has some form of "slip" built into the transfer case, be it viscous coupler or mechnical means. A true 4WD will have the front and rear locked by means of gears or chain in the transfer case. There is no slip between front and rear axles, hence no driving(or more appropriately, TURNING) on hard pack surfaces while in 4WD but an AWD is OK. The slip is necessary b/c the front and rear axle will be turning at different rates due to different arcs while in a turn.
I am guessing that the makers of the 4WD tractors factor the tire size difference and adjust the front and rear gear ratio accordingly to get rid of as much rotational difference from front to rear. Even some OEM trucks have a slight difference (like 4.11 front and 4.10 rear )but if using on a slick surface it does not cause binding.
Open, L/S, and locked axles only affect the two wheels on that axle, not make it "4wd". The transfer case is what makes it AWD or 4WD.