I just purchased a 2001 GMC Sierra with the allison transmission. It has 50k on the engine/tranny. Today when I started it to drive home, the gear indicator lights wouldn't work, the "service engine soon" light came on, and it wouldn't shift out of first gear. Putting it in "tow" mode made no difference, nor did starting the gear shift at first gear and then moving up to drive. The fluid level was fine. Any ideas?