Getting ready for a 6-7k mile trip, the tires on the truck have 36k on them. They measure 8/32nds vs 15/32nds when new (the spare). The old standby method is using a penny to show worn out tires ...
I don't know, at 8/32nds, you are nowhere near the end of the tires life. If you calculate you have 15k left, and you're only going to drive half that distance, it seems kind of silly to replace them now. And believe me, this comes from Mr. Preventive Maintenance. It would be a different story if you figured you would reach end of life some time around when you get back, but you will still have another 8-9k left!