Oxygen Sensors when to change??

I have a 2004 Sebring Touring 2.7L with almost 100 000kms (60 000miles). 
Gas mileage has suffered recently. Check Engine light has been on and I
have 
scanned it...

I have an OBDII error code of 0137.  Am I correct that is the oxygen
sensor 
on the driver's side of the tail pipe that is faulty?

I'm taking the car in to have a tyranny flush and new serpentine belt
done, 
should I just have the dealer replace all 4 Oxygen sensors or replace the 
faulty one.  The cost of the oxygen sensors is $47 each from the dealer,
not 
including the labor which was half the price from several auto parts
dealers 
in my area.

Would their be any benefits to replacing all 4 oxygen sensors??  I really 
want to get another 100 I kames (60 K miles) from this car.

Thanks for the input!!