Update:

I went ahead and replaced the flex plate and the sound went away. There were no visible cracks, just a small gap between the plate and the gear ring welded on the outside.

Also replaced driver manifold (oem), and the 2 rear cats (walker) while I was at it.

Notes - I tried to use 4 120mm bolts to slide the trans back on to keep everything aligned. I could take out the flex plate bolts, but there was not enough room to slide the flex plate out past the frame cross member.

If anyone needs a passenger rear cat, amazon had the walker california version for only $230 ($645 on rock) - Amazon.com

Thank you all for your help!