The recall covers Chevrolet Silverado and GMC Sierra light trucks built between October 1999 and October 2003. Some 2002-04 model Chevrolet Avalanche and Cadillac Escalade EXT sport-utility vehicles are included.
A spokesman said 3.7 million of the vehicles were sold in the United Stares, about 325,000 in Canada, and 93,000 in Mexico.
GM says the tailgate support cables have a tendency to rust and break. If both cables fracture the tailgate can drop an additional 10 degrees to the rear bumper. There have been 134 injuries related to the problem, but no crashes or fatalities.
Dealers will replace the existing galvanized, braided steel support cables with stainless steel support cables at no charge to owners.
"Our priorities are to ensure that our customers are aware of this field action and provide the replacement cables in as timely a manner as possible," said Kevin Williams, vice president of GM North America Quality.