You can always book a current fare, and then if a cheaper one comes out, cancel the ones you have and apply the fare in your ticketless funds to the new flights.
If you do that, one should probably have two computers going. The first one pulls up the tickets to cancel while the other one "holds" the tickets you are booking so as soon as you cancel you can complete the second transaction.
We are traveling from Phoenix, also. And, the fares are horrible ... even on Thanksgiving Day which is a slow time, supposedly. I went ahead and book free ticket flights (be aware of the changes in the SWA program -- see my thread on that) and then if a lower price comes up I will buy mine as my company reimburses me for it since I have to do some business down there during the trip -- and cancel the free one. I am hoping that they come out with some specials in the months ahead.
But, if you must have your particular dates with no flexibility, I would book it to make sure you have a seat.