If the rear end has never been modified, then the axle code on the Safety Compliance sticker located on the driver side rear door post will provide the info. A code of 13 is 4.11; 16 is 3.50; 18 is 3.08; 19 is 3.54; If there is an H instead of a 1 then it is a limited slip differential.
An additional method would be to jack up the rear end, both sides. Put a mark on the drive shaft that you can see. Turn the rear wheel one turn and count the number of times the drive shaf turns, ex. just slightly over 3 you have 3.08, a little over 4 turns 4.11. Note: with an open differential you need two people, one for each wheel and turn at the same rate.
Keep in mind if you change the rear gear ratio, you need to change the front to match it. If you don't, you will have a real nasty experience when going into 4x4.
There is a real good chart in Jeffs Bronco Graveyard "80 and up", catalog to help you pick a gear ratio that is right for the way you intend to operate the truck. I believe it is on page 30. In your case 4.11 or 4.56 is probably the right set. Also keep inmind the chart is calibrated for 55 highway cruise so if you cruise at 65 then the numbers move up a couple of notches. Jeff gives a formula to let you calculate your own numbers. If you have 3.08's in there right now that will definately make you a power Dog with 35's on it.
I have 3.08's and with my 33's on it is a power Dog. I get taken off the line by snails
) .
Good luck,
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