You can get some great deals on second hand tires to match the 75% tread left on the other rear tire. I have had great luck at getting near new tires from ebay for less than 50% of new price shipped to my door. For instance, here is a Michelin PS2 with 6/32nd no repairs for $150 shipped to your door:
http://www.ebay.com/itm/1-265-35-19-...#ht_2690wt_896