Preheat the oven to 375° and position shelves in the upper and lower thirds. In a bowl, combine the garlic powder, onion powder, pepper and 1 1/2 tablespoons of salt. Set the ribs on 2 rimmed baking sheets and rub with the spices. Cover with foil and roast for 1 hour and 30 minutes, until tender, switching the pans halfway through. Pour off the liquid in the pans.