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'm in the other camp...since you obviously have a concern, I'd go ahead and replace them before the trip.
Give me a worry free, trouble free vacation anytime over one where I worry about the tires and if ONE of them goes, it'll cost you more than replacing them all now....just sayin....Dennis