Yep, that's broken.

"I've never seen one break like this before." I almost love it when people say that. Almost. 

Apparently, a common failure point on a 4L60e is the rear planetary gear set. The gears get chewed up and bang, it's done. 

When I dropped it off he said it had a broken rear planetary. What it has was a broken front planetary. But the gears aren't chewed up. It still turns very smooth. It almost looks like the carrier exploded from the splines out. $1500 later and it has new gear sets and wearables as well as a new valve body. Hopefully, it will last a while. 

What were you doing when it blew up?  That's an odd failure. 

In reply to WillG80 :

Accelerating around a slow car. It downshifted and bang. No grinding, no warning, just bang. 

I agree, definately broken.

Other than the bit about boost, I think this picture exists for a reason:

Looks like you tried buffing it out. Usually that works.