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There are two ways to find out yours. One is the ID tag that is attached to the driver's door frame. At the bottom of the tag there is a two digit code for axle. Your owners manual will decode what the axle is. If you don't have a manual, post the code here and we'll decipher it for you. The other way is to crawl under your truck. The rear end should have a metal tag on it with the axle ratio.
Not to confuse the issue, but there were many different combos put into these trucks. I would only trust the tag on the axle itself.
a good example is my '82 f-100, came with a 232 v-6, 3-on-the-tree and 2.73 rears.