Just thinking about it further. The Caps really do surround it. It really looks like the Carrier shim but I can't see how a piece that large could have gotten past the housing or the bearings to escape and be sitting in the bottom of the housing.
I had a 0.007" difference between mine. Determined during initial Pinion/carrier Setup

 

I'm near positive that it would be a piece of a shim...it looks identical to what I recall from a couple weeks back.

But as you say, if the caps overlap the shim there ought to be no way for it to come out.

I'm inclined to think that it's a remnant from factory or a previous rebuild.

-Jeffr

 

Hey guys, thanks for all the work on thinking about this. I'm not very familiar with working on the inside of diffs but if those shims are contained by the bearing caps then I can't see how a piece that big could come through either. Also it was the only piece I found and it looks like it has a chip missing on it, but that never showed up when I drained the diff. Will pull it apart later this summer when I get more time. If anyone comes up with a way that shim could have gotten out I'd like to hear about it. Again thanks for all the input on this one.

 

Talked to a fellow who knows a bit about diffs and he suggests it might be a washer that sits behind the pinion gear. I checked the races around the bearing caps and nothing is missing. Also no shims missing inside the diff itself and I think JeffR was right that nothing from the axle will make it through to the diff. The rest of the pinion gear washer may still be doing it's job behind the pinion gear.... for now.
I traced out a full circle using the piece I found inside the diff as a guide and it's 1 13/16" ID and 2 13/16" OD. I'm attaching a few picks of the inside of the diff, my drawing, and a view of the yoke, bearings, and pinion gear from a typical setup. If you happen to know what the piece is please let me know. It still might be someone's idea of a joke right from the factory.
P.S. also including a photo of the diff with those two holes on each side that are sealed off by the gasket and diff cover. Believe they may have been used at one time to lube the axle bearings with the diff fluid?

 

Those holes are not lube holes. No need to worry about them. And the comment on GM making axles to wear out after 80k miles is baseless. Many go FAR beyond that with regular servicing.

 

The Spacer under the upper pinion bearing is usually Very much thinner then your 1/4" thick chunk of mystery metal.

 

I wasn't sure of the thickness of the spacer/washer behind the big pinion gear becasue I can't see it from inside the diff. One of the pictures I included last time showed the pinion gear and bearing setup and it didn't look very thick. Did some more searching and see those holes are for "spreading" the diff (see attachment). Had read some stories that had mentioned lubrication.
Thanks for the input guys, it's great to have this forum to learn about these trucks. A lot of people slam these vehicles and they do have their quirks but I've been a loyal owner since 1988 of 2 Blazers (1986, 1994) and 2 Jimmys (2000, 2005). With regular maintenance none of them has ever let me down unexpectedly.