Re: Help with IDing rear diff
 

It's a K series so you'll need to match the front and rear axle ratios.

It should've been stamped a little closer to the carrier mounting surface but it's still reasonably readable.

Looks like the part number is 3817778
The gear stamp looks like 11:41

GM 3817778 is a 3.73:1 gear set.

Re: Help with IDing rear diff
 

Matching the front and rear ratios is something that never even crossed my mind, thank you for bringing that up. I will probably keep the gearing exactly where it is anyway, but the thought of changing the ratio did come to me. Thanks again

Anyway, after refilling with new fluid, I was hoping the whining would diminish some, but sadly that's not the case. Just as bad as it was before. I just need a few months of twenty miles a week or so and then I'll be rebuilding the rear

Re: Help with IDing rear diff
 

Mismatched front to rear axle ratios can and probably will do bad things to the transfer case and can foul up the transmission and/or one or both of the axles as well.

If you haven't put it in 4wd without a catastrophe you probably want to verify the front ratio.

3.73 is an optional 1/2ton axle ratio. Your SPID label in the glovebox should call out an RPO code for the axle ratio installed at the factory. GT4 is 3.73 axle gears and G80 is the Eaton Gov-Loc differential.
I'd be fairly confident the axles haven't been fooled with if the SPID matches the parts installed in your rear axle... but verification never hurts. You have to pull the axle cover to change the gear oil.

The G80 does bad things when used with limited slip additives so someone that knows will likely look at the SPID and assume the Eaton is still in place. If you put a limited slip diff in the axle it's a good idea to put some kind of tag on an axle cover bolt above the axle tube noting that Limited Slip Additive is now required.
This is one of those "posi" tags from GM.
https://www.ebay.com/i/330781133705?...d=330781133705

Re: Help with IDing rear diff
 

The transfer case is a whole other thing... I took it out a few weeks ago and tried to pop into 4WD, but the linkage was seized which is a pretty common thing I guess. Anyway, linkage works now, but I haven’t had a chance to take it out and test it. It is a NP203 that I am 50% sure has been converted to part time. I’m going to jack it up tonight and see if I can spin the front driveshaft while it’s in gear. I’ll also look for that tag in the glove box, I wasn’t sure where the tag was supposed to be. And I’m sure I’ll end up opening the front diff at some point this weekend to take a look in there as well