I have a posi rear end and i cant figure out why the wheels will turn in park when the car is jacked up and not while it is on the ground. Both wheels will turn when they are both in the air and while the car is in park. But once one wheel is on the ground they will not turn. I just dont see what keeps the wheels in gear when on the ground and not when they are in the air. I have been trying to figure this out for a while and have not come to any conclusion so if anyone can explain this to me that would be great.

 

When the car is in park the drivshaft wont move so neither will the wheels. When you put both wheels in the air the driveshaft still wont move but the internals in the posi will drive the other wheel too. When you put one wheel on the ground the clutchs are preventing the wheel in the air from turning.