2006 GMC Duramax with Allison 1000: P0843 and P0877 Transmission Fluid Pressure Codes Explained

I recently encountered two transmission fault codes on my 2006 GMC Duramax with the Allison 1000 transmission: P0843 (Transmission Fluid Pressure Sensor/Switch A Circuit High) and P0877 (Transmission Fluid Pressure Sensor/Switch D Circuit Low). Code P0700 (Transmission Control System Malfunction) also appeared. The issue occurred two days after driving on extremely rough dirt roads, while I was still over 75 miles from cell service. At that time, the transmission was completely inoperative — no reverse gear and only very slow forward movement. The transmission fluid level was at the cold mark but showed visible foam. With no alternative, I cleared the codes and drove approximately one hour. After this drive, the fluid became clear and settled into the middle of the hot zone range. Since then, over 2 weeks and 50-odd miles, the transmission has operated perfectly with no recurring issues or error codes. The truck is at 81,000 miles; only about 1,500 miles have passed since a previous fluid drain, refill, and external filter replacement. What caused these codes? How can I properly diagnose and fix this issue? Thanks for any insights from experienced Duramax owners or Allison transmission technicians.