There are numbers stamped on the Differential Case (the part right behind the drive shaft). The numbers stand for the number of teeth on the crownwheel and the pinion. You divide the first number by the second i.e. 43/11 = 3.909 or 39/11 = 3.545
Basically the conversion requires pulling the drive shaft, differential (pumpkin),axles, having someone set up the new gears and reinstalling with new seals and bearings as needed. It's pretty straighforward but you should get a workshop manual to see if its within your mechanical scope.
I did the pulling and reinstalling and still spent around $700 for the new gears, set-up, bearings, gaskets, micro-polishing and shipping.
Hope that helps,
John