Tom, this is a strange one given the age/mileage & level of maintenance you've done on the truck. Has is done this since new? I'm at 40Kmi with my 2002 LS 4x4. It has pinged slightly here & there, but it's rare. I top out at about 17mpg. I hear you on buying higher oct. fuels. No way I'd do it. You've made your dealer aware of your situation, so if you have the 10yr/120K mile P-train warranty, i'd keep up on the maintenance & drive it until it craters, then have them fix it
! Historically, trading-in or selling a used 'Zu does not gain you much $
. It's possible, whatever system is creating the ping may rear it's ugly head someday by lighting up the CEL, then your problems *could* be solved! Until then, I'd just keep the radio on.
G/luck
Joel