I did that with my 99 GMC 1500 but at a GM dealer. They can give you the RPO codes for that vehicle. In there will be the RPO for what gears came with the truck.

See if this chart will help

www.sierragear.com/gm-rpo-axle-ratio-identification-codes-3/

It has come up more and more. Everyone should know whats in the truck. Below are the stock gear codes found in the glove box. You will have one and possibly two of the following codes if you have the G80 option. I have also listed a helpful gear to tire size chart to help people better understand. 

GT4 -- AXLE REAR, 3.73 RATIO
GT5 -- AXLE REAR, 4.10 RATIO
GU4 -- AXLE REAR, 3.08 RATIO
GU5 -- AXLE REAR, 3.23 RATIO
GU6 -- AXLE REAR, 3.42 RATIO
G80 -- AXLE REAR, locking differential (has many names, none being good but that brings up a whole other discussion)

Wonder what it drove like,, must have been pretty noticable!! Lol

not the ls motor in the 97 still a 5.7

My 2010 "Burb has the GU 4 [3:08"s] 70Mph & turning 1600 RPM with 6 spd. If your 97 yukon is 4x4 you'll have to change both "diff's gear ratio"!!! to maintain the tranfer case life of more that 5 min. before it screws up with 2 different gear ratios???

Find the same RPO code in the glove 2WD or 4WD as ling as the ratio is the same.

I had the same ratio in my 99 GMC 1500, 2WD, with tow package, it had the e-locker.

stax,
you have a wide range of years to look at for G80, it's just a matter of the finding the right gear set.

The below from :: http://www.silveradosierra.com/transmission/what-kind-of-diff-do-i-have-t2321-10.html#p18340

Re: What kind of diff do I have?

Postby Z15 » Mar 01 2011, 12:36pm

I was not jumping on you but trying to correct misinformation. There are often arguments over this in forums where some insist its a limited slip. I have even encountered some GM techs that do not know there is a difference, especially if they are not skilled in differentials. Believe me, I had to stop them when installing AMSOIL in my 2004 GMC because they were going to install an additive. I observed the install tech at the parts counter getting a bottle of additive and I stopped him and that po'd him off. That's the GM dealer and a tech with nearly 20 yrs of experience who was giving me the hard time.

The axle is not coded in the VIN BUT a GM can run a "build out" of any vehicle using the VIN. Ask them for a paper copy of that, don't take their word.

Just fyi - These new trucks I had were always ordered with the G80 locker
1987 Silverado regcab (my last 4x2)
1989 Silverado regcab
1992 Silverado Z71 Ecab
1996 Silverado Z71 ecab
1999 GMC Sierra Z71 ecab
2001 GMC Sierra Z71 ecab 2004 GMC Sierra Z71 ecab
2008 GMC Sierra Z71 ecab
2010 GMC Sierra Z71 ecab

These cars had the G80 limited slip
2002 Trans Am WS6 LS1
2005 Cadillac CTS
2009 Cadillac CTS4

This pic below is from a 2003 GMC brochure. I had a 2003 GMC Sierra SLT Ext Cab Z71.
Zoom in (real dimensions: 791 x 1024)Image