Stalling when down-gearing from OD to 1st (auto trans)

[Explorin'VA]

My check engine light has been appearing intermittently for a few months and recently my truck began stalling out when I went from driving in overdrive to a complete stop (auto trans). The engine functions properly when I'm driving but it will stall out at stops as soon as I hit first gear. The only way around it is to pop it into neutral as I come to a rolling stop, then pop it into park and pause, then shift back to drive. Doing this prevents the engine from stalling. To me it feels like something is wrong with the transmission, like a failing torque converter. It idles fine any other time, except when downshifting to first it surges and stalls out.

I got codes 558(EGR vacuum regulator circuit failure) and 332(EGR valve not opening). I replaced the EGR valve and the the EGR pressue sensor but neither fixed the problem and I'm still getting the same codes...

What do y'all recommend I try next?

 

[Explorin'VA]

Would a worn out flexplate cause this? Lately on cold starts the starter creates a grinding, screetching noise, but the starter isn't loose. Is it possible my torque converter or any of the transmission internals are causing my truck to stall out when down gearing from overdrive to 1st?

I need advice because if it is a transmission issue, then I'll just get started swapping in a M5OD that's been sitting in my garage.