What could cause a loud bang/chatter at cruising speed on fwy?

My Silverados 4l60e with automatic transmission suffered an unusual failure after entering the freeway & making last upshift. With no previous symptoms, it started Banging violently (felt like the rear end came apart)

After an abrupt stop I checked the rear end and driveline with the rear wheels raised and found nothing.

So I started the engine and revved after a minute or so of perfect idle. I Checked all fluids which were fine. ATF was a bit dark as a filter/fluid change was overdue for awhile. An automatic will usually let you know before it fails, so I wasn’t surprised when I finally pulled the shifter into gear and truck responded perfectly.

I rolled forward then lightly pressed on the throttle, but after 20 feet there was a CLUNK, the truck jerked & I shut it off. I towed it the 15 miles home in neutral, and there were no odd sounds of any kind.

I pulled the differential cover & checked out axles, rear end, driveline etc and all looked fine. Obviously, I pulled A/T. I even started the engine afterwards to be sure, so I’m positive the problem is in the transmission. But what could it be?