You will need to count the teeth on the Ring, the larger gear powering the axles
And count the teeth on the Pinion, the smaller gear connected to the Drive shaft
Then go here: Ring & Pinion Calculator
And enter the numbers
Front and back will, of course, be the same.
I would see if you can find an L/S(limited slip) replacement vs an OPEN, L/S came in all the popular ratios
L/S is Fords version of Positraction(GMs version of L/S, lol.)
OPEN means only one wheel gets power, and the easiest wheel to spin is that one wheel
You can look here: http://www.therangerstation.com/tech...le_codes.shtml
Get the door code numbers and hit the wrecking yards for a match.
So say you have a 7.5" 3.73 now
You would want to find a Door Code F6, L/S, 7.5" 3.73