As I recall, the Allison can be manually controlled using a Tech II scan tool. A GM tech can go on a ride-along, and select different gears as a way to test the transmission systems. Might be worth a shot.

Secondarily, the Allison has always been a "smart" transmission. The programming learns from the way thee truck is being used to modify its shift scheduling. There are ways to reset the learning process, and have the transmission begin the process again. A search here in this forum for the appropriate keywords should produce more reading and instruction. Jim