Ok, maybe this will help. Rebuild the Diff from scratch. For info, the 2 primary modes of failure for the 4T60E(HD) trans under the Series 1 L67'* is either the shaft of pin.
Look at this pic:
See that vertical shaft on the far right? It holds the two little planetary gears in the 'cage' or housing. See that little pin perpendicular to it? That'* the pin that holds that shaft in the cage.
The shaft can break, allowing the 'unrestrained' (unpinned) half to come free, or the pin can break, allowing the whole shaft to come free.
That'* what happens in a nutshell. Keep the cage (never seen one fail, although mine is new), and replace ALL other parts you see in that pic above. I have most of them in a box here from my last failure.