How to get the differential ratio on a Ford F-150 4X4 rear di

I blew the back diff on the side of the road I had to take the back cover off and remove the gears so I could get the remaining 2 miles home I have the gears out and was wondering how to get the ratio of it whether I have to count teeth etc before I try and order one from an auto wrecker so I do not end up with 2 different ratios from the front diff to the back any help would be greatly appreciated thanks