PROBLEM SOLVED ..... I think.
After all of the discussion above and some extensive digging on the internet, I went to the garage with a full checklist of things to check.
I pulled the inspection plate, and dust cover.
Saw no evidence of contact anywhere. I noticed the transmission side of the ring gear teeth had a bevel on them, but it seemed too uniform not to be from the factory.
The inspection did cover had a dent in it about the size of an egg, approximately 1/8 inch; deep. But no signs of contact on the cover OR the torque converter.
Just to be sure, I worked out the dent on a block of wood.
From there, I decided to check the torque converter bolts.
They all registered as torqued to spec according to my torque wrench, which at this point I have LITTLE faith in.
But I cranked them down a little more.
Took the Jeep out for a ride ... took off in first, quiet as always, shift into second .... Nothing. Completely quiet.
Turn around and same thing .... First was fine. Second ... quiet. Shift to third, nothing.
Ping/ knock/ clatter/ whatever ... GONE!
Unfortunately, it was getting dark and I hadn't reconnected the lighting harness (in case I'd have to pull this engine again).
So I couldn't take it back out after reinstalling the covers, to see the result there.
But for now, it's gone.