Dealer traced back to the ecm, despite them telling me these were on a long backorder wait list, it arrived the day after approving the quote.
The service advisor told me that the truck was in 95% power reduction limp mode. Which equated to ver rough, low rpm idle. It would move at about 1mph which was great because it needed to move to get it in a place for the tow truck.
Thinking back maybe I had some telltale signs. Intermittent tcm j1939 communication errors, intermittent low oil alarms and intermittent outside air temp readings.
Based on the drive home, the downshifts seem crisper. Idle seems better and throttle response seems better than before it went into limp mode.