Front or rear diff, and why do you need to know this? (as I'll have to tke a cover off one to count them), maybe I can give you advice without doing this ?
Obviously you must use the same Front and Rear on a 4X4 Car.
There is a 3.9 ratio which is from the 2.0 Sierra 4X4 which I am using in my Puma 4X4 Front 3.9's are getting hard to find now

HTH Dave