NEVER ENGAGE 4WD ON ANYTHING BUT LOOSE TERRAIN!!!!!
Yes, you are correct, you have open differentials front and rear. What you are missing is a "differential" of sorts in the transfer case, as an AWD has. Our t-cases lock the front and rear axles together as a unit. It does not allow slippage between axles. I.E., if you turn, your front axle makes a much bigger radius than that of the rear, meaning the driveshaft spins more. Since your front and rear driveshafts are locked in together in the t-case, there is no other way to release the binding than tire scrub, other than tires breaking loose. In AWD, there is an additional "differential" device in the t-case.
Rob L.