You are checking it wrong, this is how you check open differentials,
Like previously mentioned, if your car has an open differential (non-positraction), you will want to perform the check with one rear wheel on the ground. Again, mark the inside of the tire and a spot on the driveshaft with your paint marker. Now, rotate the suspended tire two revolutions and count the number of driveshaft turns. The reason to make two revolutions is the open differential actually rotates half as many times as a ‘posi,’ because of the design of the “open” gear operation
Spider gears make one tire turn twice as fast if other is not.
