Here's a little update. I finally had a chance to dive into the O2 sensor. It was failing silently so it was not throwing a code. Until recently. A few times the engine light came on then off. I replaced it and sure enough the O2 sensor was responsible for 95% of my issue. There's still a little something going on but it's incredibly minimal and may be due to residual or learning issues.

I'm going to replace the other matching O2 sensor as well since they tent to fail around the same time. But with 255k on the clock, with original O2 sensors, I'm not complaining. I'll b replacing all of them over the next few months but at $150 a pop I'm going to take my time.

Crazy that the O2 sensor could cause such an issue to feel like my transmission was dead. I learned something new.

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2008 X5 3.0si
256k Strong