"Stellantis has the worst proposal on the table regarding wage progression, temporary worker pay and conversion to full-time, cost-of-living adjustments (COLA), and more," the union said in a statement.
The Sterling Heights Assembly Plant north of Detroit is Stellantis' only source of the current-generation Ram 1500.
The abrupt move is the UAW's first expansion of the strike since Oct. 11, when it added Ford Motor Co.'s Kentucky Truck Plant. That plant builds Super Duty pickups as well as the Ford Expedition and Lincoln Navigator large SUVs.
More than 40,000 UAW workers are now on strike as the targeted work stoppage nears the six-week mark.