Engine stalls when put in Drive?

I have the 5.3 flex fuel engine. About a week ago, the truck started having an occasional issue where when you hit the gas, the engine would rev, but the truck wouldn't accelerate for a few seconds, like it was slipping. Sometimes it's like it builds up and then finally catches and launches you forward, other times it feels like it's in too high of a gear and won't downshift to the correct gear.

Then, it started occasionally stalling when coming to a stop as the RPMs would drop to idle. That has now turned into the engine stalling every time I put it in drive from park. If I put it in reverse, it's fine, but as soon as I drop it down to drive, the engine stalls. If I keep the engine running by feathering the gas then it will stall when I come up to the next stop. Placing the transmission in neutral just before I stop will keep it running and then back into drive with a little feathering to go again.

My first thought was fuel pressure so I replaced the fuel pressure regulator. Static pressure (KOEO) while the pump primes is about 48 psi, when it stops priming it's about 40 psi. Running pressure is also about 40 psi, pulling the vacuum line from the regulator while running jumps the pressure up to about 46 psi.

There were 2 pending codes, bank 1 rich and bank 2 rich, but no stored codes. I have a mid-level scan tool that is bidirectional and can do many active tests and special functions. I did a misfire count test, but all were zero. I also did a transmission slip test, which also was zero (it's possible I didn't do it correctly).

I've clean the mass air flow sensor and the throttle body. With there being no stored codes, I'm not sure where to go from here.