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in your glove box, there should be a sticker with a whole bunch of 3 letter/digit codes, known as RPO codes. They list the options/features in your truck.


It'll have GT4 or GT5 (most likely GT4) indicating you have a 3.73 ratio.


If you want to know info about the rear axle itself (like if it's a 10.5" or 11" or whatever), the RPO codes don't have that level of detail, someone else should be able to tell you what it most likely is, or you can look at the bolt pattern of the diff cover and measure it to figure out what size it is.
 

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...or maybe G80 for the "gov-lock" differential. Put that in the search and you can read arguments about this one for days!!
 

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Discussion Starter #5

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The Duramax should have enough power with that rear end to pull your load up a telephone pole (if you could get traction, but might only be a small exaggeration :))

[added on edit] And that's already a tall gear for highway use, unless you want to go really fast all the time. Unless there's something wrong with the diff, it's best to leave it as it is.
 
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