Some suggestions, code P258B and related problems

[Shake N Bake]

Truck is a 2021 Power Wagon 75A. On a recent trip, I had three instances while in 4LO where my brake pedal went to the floor. It felt as if I lost all vacuum brake assist. Conditions at the time this occurred, extended low speed downhill crawling. This was not a good feeling while negotiation switchbacks in Colorado. Once back to level ground, I switched back from 4LO to 2 wheel drive. On paved roads, the truck would not shift past 4th gear. Truck was revving high and it felt like I was still in 4 wheel drive even though it showed me back in 2 wheel drive. Fuel consumption was 6mpg on the short drive into Telluride. Once there I checked for codes and found P258B stored. I cleared the code and did a full reboot using Tazer. Once I did this, everything was fine, truck shifted in all gears and fuel consumption was back to normal. I was able to replicate the shifting issue once again on a dirt road, and again once I cleared the code and rebooted, everything was fine after that. Ive done some searching, and the code P258B seems associated with a vacuum assist pump. How / why would this affect my shifting / 4 wheel drive to 2 wheel drive system? Anyone have the location of the pump and tips on trouble shooting this? Id like to do my homework before it goes to the dealer. I get that low rpm crawling would cause low vacuum, but once back up to speed vacuum should improve and the truck shift normally again, right?