Well based on the application (king pin shims), I would assume they are relatively hard and would be suitable for the differential application. You can get a quick and dirty idea of how hard they are using a small hand file to see it it will cut those new shims. The corner of a small file will a) cut like butter, b) completely skate across the shim without cutting, or c) somewhere in between. You want "in between".
Another thing you could try is if you fold one of those shims in half... Does it fold easy and stay pretty much completely folded? Does it snap into two piece? Or does it fold but springs back a bunch when you let it go. I think you want the third option.
I know this is all just subjective words, but from a distance, that's all I got.
And I may have mentioned this already, but I can run a Rockwell hardness test on parts if necessary to determine the Rockwell rating. Although I'm not sure it will be accurate on something as thin as .005.