5L40-E Hard Shift 4->5, dark oil, BMW code 37

[Esgra]

Hello all! I am new to the forum, but I have been using it for at least 8 years as a guidance to repairs on my '05 X5.

About a year ago my transmission hard shifting from 4th to 5th speed. It has caused a couple failsafe modes, and it seems to be isolated to hard acceleration, especially when the engine is still relatively cold. It also makes a "squeal" when this happens, which quickly resolves (about 5 sec). I thought it could be something innocuous, so I changed the fluid and filter as a starter. I used ACDelco synthetic Dextron V ATF (it is a GM transmission). After changing the oil, the symptoms did not alleviate, so I double checked that I added enough oil.

When changing the oil, there was some metal stuck to the pan, but it was a small amount (sort of a slurry). However, when I checked the oil (probably a month later), I noticed it was very dark.

About 3 months ago, I drove the car and noticed a spontaneous downshift from 3rd to 2nd, but I could not repeat it. I was also able to record a BMW code 37 on INPA (Gear Monitoring 5).

Anyone have any idea what the issue could be? Right now, I do not drive much as I have the car in a storage locker. I imagine I am looking at a transmission overhaul. If so, does anyone know what I need to overhaul (just the valve body, or the whole works), and where I can find parts?

I am willing to rebuild it when I find time, and I will write-up my work as well.