If you can't figure out how to read the code from the door jamb/online.

Mark tire with chalk at top center.
Have friend drive forward SLOOOOWLY with friend watching tire chalk/verify 1 revolution.

You sit next to the truck and count how many times the rear u-joints spins.

I didn't have a door code
Mine span alllllllmost 3.5 times. I have 3.45 gears

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'83 Ford Ranger 2.3L 4Spd, Dual Tanks, 195/60R15's, Welded Rear End TOTALLED AT 483,211 MILES