get your owner manuel out. look at the VIN number on your vehicle. then in handbook read how to translate the vin number.vin number has the information you need also should be a tag attached to one of the bolts on the rear end with the ratio stamped on that tag
if you have the owner handbook, it will tell you how to read the vin .that will tell you what rear end you have
a chev dealer can use the VIN to tell you what rear end is in your dually. also if you have the owners manuel for the truck, it will tell you in the book how to tell which rear end . should also be a small metal tag n the diff housing with the rear end specs written on it this tag could be missing if not the original rear end
You will need to remove the plate that's on the back of the rear end housing and look inside at the gears to know for sure.
you would have to check the code in the glove box and call the local parts store and have them tell you what rear end goes on the Yukon and then find out what rear end the 1990 rear end is and if they are the sam gear ratio then they ar enter change able .
Simple answer is count the bolts on the diff cover. Generally 7.5 rear ends are 10 bold and have somewhat square corners. While the 8.5 rear ends have 12 bolts and are round.
There will be a small metal tag on the rear diff. It will tell you your gear ratio, and will have an "LS" after that number if you have a limited slip.
If you have to ask, then you'll never know
not exatcly sure about the 45 but I can tell you about a 37... the fill plug for the rear end is on the rear cover to the right and about two inches above the pto shaft and you will fill the rear end to the level of that fill plug.
Jack up the rear end safely with both rear wheels off the ground and try to turn one wheel forward while in park. If the other wheel turns rearward you have a "one wheel peel" rear end. If you can't turn either wheel it's a limited slip.
The rear end gear ratio was 3.55:1. Optional was 4.10:1. You can tell what rear end you have by looking at the label located in the glovebox or on the underside of the hood at the front in the center. There should also be a label on the center differential as well.
Get your rear-end waxed.
get a posi traction rear end and swap it out.