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What year/model is your truck?
The VIN will tell which gears came in it.
If they have been changed, easiest way is to pull the diff cover and count ring and pinion teath and divide number of ring gear teeth by the number of pinion teeth.
If the tag is still on the rear differential that will tell you as well.
I think you can tell by the tag on the door jam too.
Its either 3.55's or 4.10's
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