O2 sensors In a 96

Well I need to do a tune up, Im runing about 10mpg but thats more do to 4.56s and the 34 LTBs. I have never re placed the O2 sensors, b/c its not giving me a check engine light or throwing and codes.

SO, should I go ahead and repace them?
Which 1s?
Where are they on a 96?

Thanks guys

The upstream O2 is the one that will give you horrible mileage. The downstream is to check the cat function. You definitely should replace it. If I remember correctly its just about under the center console in front of the tranny. After I replaced mine I went from 11mpg city to 15mpg city and 13hwy to 20, its definitely worth the 60 or so bucks for the Bosch too.

thanks alot