One problem related to automatic transmission torque converter has been reported for the 2003 GMC Envoy. The most recently reported issues are listed below.
I have a 2003 GMC Envoy, 80,700 miles original owner. While merging onto highway transmission locked in 2nd gear. Engine rpm's increased to 5,000 speed dropped to 20 mph creating a dangerous situation with oncoming traffic. This happened without warning or any indication of a problem. I had vehicle towed to GMC dealer, asked if there were any known problems with GMC Envoy transmissions and was told there were none. The diagnosis, my transmission exploded internally with a failure of the sun shell gear and torque converter. Transmission could not be rebuilt and the cost of installing a re manufactured transmission was $3700! gm has been having these transmission failures since 2002 with no acknowledgment of liability!.