97 Safari w/ 4.3 Auto A/C

I have been getting crap mileages on this vehicle, tho I've done all of the regular tune-up replacements, including wires. It runs a bit rough too. I have been told that if I swap out the old o2 sensors, I could probably grab a possible extra 5 mpg.

Could anyone tell me where the o2 sensors are located on this one, on the engine side of the converter. I can easily see the one behind the converter, but can't see the one(s) in front. Are they a big deal to change time and effort wise?

Thanks in Advance ... Mark