s-10 rear end balancer?

OK, I'm clueless. My wife's friend (who is murder on cars and can break anything!) thought she had a u-joint go out. Nope, they're good, but there appears to be a balancing ring on the nose of the differential, and this is flopping around like, well, a loose balancing ring on the nose of the differential. My local auto parts store doesn't list a replacement part, rockauto doesn't, Napa doesn't. So, does that mean that this part never breaks? Is it possible to replace this, say, for instance, by removing the nut on the front of the yoke, put a junkyard balancer on, and then just tighten the nut back down, or does this mess up all the clearances inside?

Did you check with a GM dealer? I believe by your description this is a damper not a balancer. Has nothing to do with the internal settings in the diff.