why does the driveshaft..
 

rotate about 45 degrees and comes back when revvin it in park?

um, I'm assuming you are up in the air with the wheels off the ground?
there is a pin holding the output shaft from rotating, but it does not lock it in place, it just keeps it from going beyond a point.
if you revv the motor in park, the fluid and rotational mass will affect all items not locked in place.