full fare ticket = y fare?
if so, it's refundable. go to an agent, tell them to tag the segment as INVOL, rent a car and drive to SPI. then go on united.com with your eticket number and request a refund for the ORD-SPI segment.
you can also request to be put on standby for the sold-out ORD-SPI flight later today, as a displaced y fare passenger you have a decent shot at clearing the list.