For non-posi, jack up the car, hold one wheel (don't let it rotate). Turn the other wheel (while the car is in neutral) through TWO (2) revolutions while you count the number of revolutions of the drive shaft. (Easier to do if you make a mark on the pinion flange and the rear end housing with a piece of chalk or something.) You'll get a number really, really close to your gear ratio.
For posi, turn both wheels through ONE (1)revolution while counting the turns of the driveshaft.
