Fixed! I had the bearings on hand as spares (thank god Kyosho uses the same bearings in the hubs) the cup and gear were fine.

So this chit, 2 $1 bearings were the problem all along!?

Just gotta put the diff back together, and shim it.

The bearings were so bad, if you shook them, the centers moved up and down, probably 1/4 the way to the edge.