<font color=blue>But I do know on a car or truck, 4 wheel drive just means one of the rear and one of the front is pulling at any one time. That is why they have all wheel drive out there now, then its all 4 wheels is pulling at one time.</font color=blue>
On 4 wheel and all wheel drive vehicles, the wheels pull the same. The difference is a 4 wheel drive the driver can select between 2 and 4 wheel drive, and an all wheel drive vehicle is always "in" 4 wheel drive.
What makes only one wheel on the front and rear spin is the "open" differentials in the axles. These allow one wheel to spin faster than the other when cornering. When the power applied to the axle overcomes available traction, generally the wheel with the least traction gets more power applied to it and it spins, which makes it look like your 4 wheel drive is only two wheel drive. A "limited slip" type differential does just that, it "limits" the amount of differential action, providing power to both wheels under most conditions. A locking differential like most tractors have on the rear mechanically locks the axle shafts together so both have equal power available to them. You wouldn't want to lock your differential on dry pavement and then try to make a tight turn. I'm sure there are others out there who are more mechanically articulate than I and can explain this better.
