A couple of times over the past several years I have been denied a J seat on this route. Since the ORD-LAX and ORD-SFO have often been continuations of flights from BRU or CDG, when those flights are full of paid J, those J pax get all the J seats on the domestic continuation. Rare, but it has happened before. Keep calling, and check at the gate at check-in. Good luck.