I had a flat tire on the front passenger side tire on my ML320.  My wife
didn't know about it and started driving, not realizing the tire was flat,
and a few minutes later came back and told me about it.  I'm not sure how
far she drove on a flat tire (never a good thing to do).  Anyway, I
temporarily put the spare donut tire on the car and took the wheel/tire to
Sears.

I told them to just fix the flat.  They said they couldn't because the tire
was damaged from driving while the tire was flat.  Ok, then I said replace
the tire with a new one.  He said that's fine but they could/will not mount
it for me because the car is 4WD and the tread depth on the one new tire
would be som much more/taller than the remaining 3 that it could damage the
gear box.  They are not authorized to put just one new tire on a 4WD
vehicle.  I then said, ok give me two new tires then and put them both on
the same axle.  Again they said no as they could not - for the same reason
about the gear box stuff.

So I eventually caved in (perhaps I should not have) and purchased 4 new
tires for the car.  I did not want to do that as about 18 months earlier I
had already purchased 2 new tures for the rear.  So $600 later, instead of
$125 later I have 4 new tires....

Does this sound right?  Did I waste my money buy having to put 4 new tires
on the car to have the same tiure tread depth to not damage the gear box?
I'm thinking of calling Sears and discussing and maybe complaining to them.
What is the deal on a 4WD car and tires with different tread depths?

Thanks,
Walter