Probably on about 8 out of 10 that I do they would NOT work. I see no need to try and reuse a factory shim as cheap as the selective packs are, especially when you buy a install kit.

Get the selective shims like M said, they are easier to work with and you have total control over backlash and preload.

 

How is selective shims going to have an effect on preload?

 

You can never be over-prepared. I have never reused diff shims, and have never been in a rear where I thought I could. It only takes a few thousandths to be off on preload and backlash. I have reused pinion shims before, many times, but that is only after taking measurements to see where I am at.
Everything that is in a basic install kit is the MINIMUM you would need for a gear install. You would not need the pinion seal, nut and shims. But, the kit would still be cheaper than buying individually.

 

You said "Never been in a rear" Seriously, with all the extra parts I need to order, would it be easier and close to the same money to just buy a brand new eaton posi? Wouldn't it come with all the shims, bearings etc.... that I need? Or would I still need all these parts with a NEW eaton? I'm not changing gears or anything fancy, just a better posi so I don't destroy the rear end after the radix get finished.

 

That just reminded me of Beavis and butthead.
The new Eaton would not come with any of the needed parts, you would have to buy all the same stuff still. If you can get a good deal on a good used unit, then I say do it.

He he, he he, I said "do it"