[big gear head]

Just because you can turn one axle and the other one doesn't turn doesn't mean it's not positive traction. I've been telling people for more than 30 years that this is a completely unreliable way of testing a rear end. You will have to remove the cover to see what is in it. Look for a code stamped into the front of the right axle tube. The casting number that you posted is a common casting used for all Camaro and Nova 12 bolt rear ends that year. The stamped code will tell you what it is.