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There's a tag on the axle, on one of the bolts, take the tag off.
You should see a 3.55 or 4.10 or a similar number on it, on my 1990 Bronco it was the bottom left number 3.55, you can see a picture of it in MyGarage, under DonKo.
Also there's a tag in the door jam that has an axle code, look up that code in a Haynes or Chilton or a 1985 Service Manual.
if it has a ford 9" then it will be 3.50's and if its an 8.8 with a 302 or 351w it will have 3.55's this was the most common ratio. if you have a 300-6 some had a 3.08 gears also