2008 silverado 2500HD extra cab axle ratio
 

How do I identify the axle ratio?

im not 100% on these trucks but i know on the older ones you can do it like i posted HERE. the reason im not sure about these trucks as i dont know what axle ratios they came with. im mainly posting to get corrected if i am wrong

The easiest way is to check the RPO sticker (on door, door jamb, or glove box etc.). Looks for codes starting with 'G' (for example GU6). Look them up here:
GM RPO Lookup

Rpo codes are going to be found in the glove box and that's only good if the glove box sticker is still there, or hasn't been changed and if the rear axle hasn't been swapped

Good point.

Sometimes the axle tube will have a code stamped into it which you can look up. Could be hard to find under the rust though.
http://astrosafarivans.org/bb2/downl...1726&mode=view

You can also open up the diff, and the number of teeth will stamped into the ring or pinion gear.
https://cimg2.ibsrv.net/gimg/chevrol...60492c676f.jpg

http://astrosafarivans.org/bb2/downl...1722&mode=view

Or turn the wheels by hand like in the link Irish provided.