Mike, I just went through this, Dan the man at my local Oreilly's hooked me up. It is called a pre-load washer for the spindle bearing kit, I had to contact my local gm parts guy at a dealership and have him look ( they really don't like to have to do any work ) in the computer for a diagram of the axle breakdown, he could order for me, so I turned it over the the friend who's truck I was working on and he got it.
Hope this helps, but that is the only place I could find it.