8 3/4 rear end worn out or not?

Yeah i have done that already, although tires spin in opposite directions, when the rear is jacked up. Thats why i am puzzled if its worn out or not. My driveway n street have proof marks.(not other car tiremarks) street has new blacktop. When i hold the brake, n give it a little gas, then release the brake it leaves two marks. So is it some other type of "traction device" or is it jus a worn out sg unit? I was told it was out of a 72 charger when i bought the car.. Its a 489 case, from wheel to wheel its 52 inches( inside of wheels) if it helps any.. They spin in opposite directions, when lifted off the ground, but they leave two marks not only one, so what does everybody think is going on? Thanks for any replies..