To all you tire gurus out there -- thanks! Today I yanked my wheels off (and the spare, all with their at least 4 yr old Marathons), took them to a local tire joint, and had new Marathons (from TireRack) mounted and balanced with steel valve stems. And all because you folks have provided an inordinate amount of detail up here surrounding tires of all things.
Anyway, thanks - these forums are a blessing.
John