I usually try to book one way flights with SWA, reason being I can change either direction and get better rates. I just did it yesterday, the outbound flight was cheaper than I had, but the inbound flight ,for my times, were unavailable. So I rebooked the outbound and saved $60.00.
If I had a round trip ,I would not have been able to do this.
If I were you, and you are sure you are using SWA, I would book the flight you can now, and wait for your return flight.