I had a grace frame, but a different model. Are you talking about the black tracking where the wheels of the carriage ride on? On my frame, you had to totally disassemble one of the sides, then the tracks slid out. But I'm not sure how you would replace them with metal rods as the tracks are specially molded to fit or if replacing them would make a difference at all.
Before replacing anything, I would check to see if the frame is level and square. Make sure the table portion that the carriage rides on is not bent or otherwise damaged and the carriage is seated correctly. Did you contact Grace with your problem?