How to identify gearing and differential size

[Mooseman]

So you don't know what diff or gearing you have, step right in. This will help you identify your differential gearing and ring gear size.

Thanks to @Capote and @Sparky , I am reposting this info for access to all and inclusion in the FAQ.

From Capote:
To find out what gearing you have and if you have a locker, the easiest way is to just open your glove box and look at the RPO codes.

RPO:
GU6 - 3.42
GT4 - 3.73
GT5 - 4.10
G80 - Locking rear diff.
G86 - Limited slip diff. (mostly on AWD trucks like the 9-7x and Rainier)

To identify the ring gear size, this chart, which identifies all GM diffs, will help:


And Sparky added this:

The covers on the GMT360 8.0 and 8.6" rear are the same bolt pattern, but the 8.6" cover is deeper. Also, the 8.6" housing is a little bigger and has an extra gusset towards the front of the housing, IIRC. If you have one of each look at the housings to compare and you can then spot the differences. I had my old 8.0 and the new 8.6 side by side during the swap and noticed the differences, but I neglected to write them down specifically or take pictures.

 

[mrrsm]

And in the off chance that you bought a Used Truck or SUV that may have changed hands several times after having been heavily modified ...with no direct references noted when the vehicle changed hands... Brian from Bransmobile1 has an interesting visual method to determine what gearing components might be hiding down there: