Two problems related to transmission gear slipping have been reported for the 2003 GMC Sierra. The most recently reported issues are listed below.
After driving over about 40 miles at highway speed the transmission will begin slippping in od. Rpm slowly climbs to about 3200 (normally 2100 at highway speed) with no change in vehicle speed, and eventually the 'transmission high temperature' warning appears on the dashboard display. Shifting to neutral and allowing the engine to idle for about 20 seconds and then returning to drive will temporarily clear the problem, but it sometimes returns.
Consumer complained about a couple of problems. One was transmission. While driving at various speeds gear slipped. When vehicle was taken to dealer for service mechanic determined that the cause was low oil. The other was for the steering column. When driving at various speeds vehicle knocked and squeaked intermittently. Steering column had to be replaced once before for same problem.