Tires: vibration, michelin tires, army tank

Question
hi..i have michelin tires, which were bought in dec. of 2007. i have taken very good care of them; very good with the pressure and rotations. so now, a few weeks ago, going home from work, one of the tires lost the outside tread. the tire dealer said this is from the age of the tires. so i had to pay for the one tire, but lately when i drive the car vibrates like an army tank. so my question is, despite the fact that all 4 tires look good, and the pressure is correct, could  the vibration of the car be the tires telling me they need to  be replaced?  thank you!!

Answer
James,

First, it sounds like you only replaced a single tire.  If the problem was the age of the tire, why didn't you replace them all?  Are they all from the same time period?

Second, as a tire fails, it will go through a series of steps - and the first step is vibration - small at first, then gradually getting worse over the course of several hundred miles, before it fails catastrophically.  

So I'm wondering if the old tires are starting to fail.