8 3/4 rear casting 1820657

I'm sure he doesn't have the entire C-body differential. Just the center section. A worn out Sure-Grip could spin one tire, but both will still spin the same direction when off the ground. If an open diff, you need to keep one wheel from turning, turn the other wheel for 2 revolutions, and count the number of times the driveshaft turns. That number will be your gear ratio. The reason you turn the wheel 2 times is that the spider gears are walking around the side gear of the fixed wheel.