just a heads up as i've read and read and read and couldn't find a complete solution.

the easiest way to determine gear ratios without looking at the actual rings (or trusting a door label):

1. mark your tires and drive shaft w/ tape.
2. spin the tire 10x while counting the number of drive shaft (DS) rotations
3. in my case the DS rotated 21.5x
4. (the part no one seems to tell you): because our trucks have a 2:1 spider gear. you must double the # of DS rotations; hence 43.
5. because i spun it 10x, i move the decimal place one to the left = 4:30 gears.

turns out the ahole who sold me the "matching" fr diff for rear axle was FOS. they didn't match at all. my front is 4:10.

hope this helps someone and saves them some time.