"Stuck" in Ohio - Workhorse W22 stalls going into gear

I doubt you have one but if you have a vacuum power brake booster it could be bad and causing a large vacuum leak. You probably have a hydraulic booster though.

Based on the symptoms the next logical thing seems to be the transmission related. The Allison transmission uses a separate computer from the engine, so it is likely your scan gauge won't read the transmission codes.