Thanks brockstar. Great answer.
His method is sound ONLY if both wheels turn the same amount when the shaft is turned. They rarely do, because of uneven brake friction side to side. If not a locking diff, raise only ONE wheel off the ground, count driveshaft turns for one tire revolution, then divide the number by 2. A 4.10 will give 8+ driveshaft turns.
If a locking diff, and it's clutches are not slipping, you can't turn one wheel with the other on the ground. That's how you check the clutch breakaway torque. Have to lift both wheels off ground.
All of the above: Transmission NOT in gear or park.