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There should be a sticker on the driver's door jamb, it should have a transmission code on it. The rear end should have a tag with some kind of code on it, there should also be a tag on the transfer case. Someone here should know what your codes mean. What year and model of truck do you have?
The info you seek should be on the vehicle certification label on the drivers side door post. The transmission, gear and axle codes are there. The transfer case and the axles should also have metal tags on them. Part numbers are stamped on them. The gear ratio codes are explained in the owners manuel eg. code 19 is 3.55 in a Ford 8.8 3800 lbs axle, an H9 is 3.55 traction lock in the same axle.
1992 F150 Supercab SB 4x4 302 AOD 3.55 31x10.5 All Terrain on Ford 15x8 Ralleys
Usually, if you post the year and model of your pickup, we can give you a pretty good idea as to what you have. Also, knowing if you have an auto or manual tranny can help on that aspect.