I just picked up a GSR tranny and i was cleaning it and looking it over. I plan to take off the case to check the gear ratio and to make sure it is a GSR tranny and not another LS. But when i was cleaning it i noticed that even when in neutral when i spin the input shaft the differential spins.

 

normal...........the rooler bearings under the gears impart some of the friction, thus the diff will spin.

Insert a screwdriver, locking the diff pin and the input shaft should still turn, with a little more drag.