driveshaft problems

yes i set the centering pins in the middle hole on the diff. and its the original driveshaft. i have the car jacked up on jack stands on the body not the rear end could that be the problem? it didnt stick out when i first jacked it up before the swap??? im confused could i move the diff. forward on the pins? or would that ofset the way it rides? both rearends are the 9-bolt pattern