The problem with the engine braking system was causing the transmission to shift back and forth between 5th and 6th gears repeatedly while at normal highway speeds, and when you remove your foot from the throttle, it would begin downshifting and causing the engine to rev very high while using the engine to slow the vehicle. This was occurring with the transmission in normal mode, and on normal "flatland" driving. The information center would also show the message "ENGINE BRAKING ON". I took it in to the dealer and at first called me and said there was nothing wrong with the truck, as it hadn't shown any codes. I re explained the problem and offered to come back in to drive the truck for them, but they said they would drive the truck again and let me know. They did get the truck to "act up" as they explained, and the fix ended up being a faulty brake position sensor, and an ECM that would not accept attempted reprogramming. They replaced the ECM and the brake sensor, and reprogrammed the ECM. Took care of the issue and drives perfect now.