So you're turning only one wheel ? Which will likely result in a 2x wrong reading ?

So it's more likely you have a 4.2 or thereabouts ?

Most diffs have an ID tag on them to indicate ratio. Or if you can open it you can count the teeth on both.

Or if you want to test via the method you describe...both wheels must be in the air and free to rotate, and ideally you'll turn the input flange on the diff and count rotations until a single full rotation of both wheels.
Both wheels must be free to rotate.