While traveling on the freeway at 65 mph, truck lost all tire pressure due to a ruptured wheel cast on drivers side rear wheel. Vehicle did not impact any significant potholes or other items on the roadway. Chime for low tire pressure went off, vehicle became extremely unstable and fish tailed side to side while slowing to a stop on the side of the highway. Replaced the wheel with spare tire and took rim to dealer for manufacture warranty. Dealer denied claim from lack of coverage. Claiming wheel must have impacted something. Contacted GM representative at customer support directly and took wheel to another dealer and had GMC district representative come out to inspect the wheel. District representative said same thing. Wheel damage cause by impact not covered under warranty. Took wheel and tire back into possession for possible future lawsuit.Replaced all four wheels with after-market manufactured rims for safety.