ah, open diff.. never thought about that.. lol.
generally if you lift both rear wheels, and spin the drive shaft.. both wheels will turn, but you can get varying results if they aren't turning exactly equally.
SO... leave one tire on the ground and spin the driveshaft and count the turns to spin the one tire, twice.
If I am wrong someone lets me know.. but with one tire stuck, the other spins twice as fast.. so you would have to spin the tire twice.
To be sure, you could have a friend make sure both tires are in the air, and spinning at the exact same time... I guess open diffs ARE tricky.. LOL...
I am pretty sure it is the leave one on the ground and spin the tire twice thing though..