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I will argue pinion depth is more critical than putting in a .036 to .042 and going.
If you have a setting gauge that will give you starting point.
If you dont , the production shim dimension is a good starting point.
Pattern will guide you from there.
If you are dealing with a quiet GM diff and putting in factory gears , I will concede you could more often than not use the same pinion shim dimension the diff had, BUT not always.

Personally I find back lash and case preload the easy part.

Depending on type of gear set you get ( two cut vs 5 cut ) pattern and recomended backlash will be different.
Also it is important to note 2 and 5 cut gear set patterns react completly different to adjustments.
Also amount of gear ring drag when reading a pattern will completly change the pattern you see

I am also of the opinion that for a 8.5 GM diff , anything but GM pinion shims are junk!
They don't fit and center well, you typically have to use multiple shims and with the 8.5's penchant for smearing the shim, multiple thin shims are , in my opinion, looking for trouble.
Ever notice the 8.5 shim is always worn ane hard to read the size?
Personaly I like to put the shim behind the rear cup.
 

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The pinion shim for the stock gears is usually .036. It is good to measure the shim.before starting. You will probably find that the shim range is within the .036-.042 range. There is a kit for shimming the rear race, and that would be simpler than putting it behind the rear bearing on the pinion.
 

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I bought a Yukon diff last week when Amazon was pretty much giving them away. Found a non greedy eBay seller with a brand new 885 motive gearset in 3.73 for $140 and Ratech sells the install kit and smart sleeve at a lower than expected price.

Who knows when I will get around to installing it all though.
 
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