My buddy's 03 Chev HD, 6.0 with the auto trans. His transmission is acting up. He disconnected the battery Sunday night to remove the snow plow harness, and when he drove it monday morning to work, he realized that it's stuck in 2nd gear? anyone know what might be up with that?

Hookem

2nd gear is the default gear. It could just be a fuse, if not there will be a code stored.