Newbie 8 3/4 rear end questions???....

K so jack one only,and doug 371 u say an open,ima lil differential dumb,and dont understand what goes on back there,why in the air do they spin?,but on the ground its a one wheel wonder?could it be worn or just an open,i was told it was a posi rear,from 72 b-body,but then again man i bought it from was a lil shady,so ill check tomorrow,thanks doug,and you too trebor 75...