When you replace O2's you MUST replace them all at once, you cannot just replace one.
Now with that being said...a misfire can set off a O2 code. The cylinder(s) that are not firing dump their fuel down the exhaust "flooding" the O2's and thus setting off a bad O2 code.
So if the trk is still running rough you must get that fixed before you replace your O2's.
A misfire will reduce your MPG, you are not buring the fuel in that cylinder so it is going to waste and the other cylinders have to work harder to take up the slack of the one not working.
Also bad O2's will reduce your MPG. Without going into to much detail,the O2's tell the ECU how much fuel is being burned and then the ECU adjusts how much fuel the injectors inject into each cylinder based on MAF readings, throttle position, gear you are in, engine load, throttle position, ambient air temp, your MPH, O2 readings,etc. If the ECU gets bad O2 readings, such as too lean, the ECU will dump more fuel into each cylinder, BUT, a misfire will fool the ECU everytime.
Does this help?

 

Thanks for the comments and education on O2 sensors. So the problem with O2 sensors is really a sign rather than a cause? I wanted to try out the suggestion of letting the truck sit with the key in the accessories "ON" position for a few minutes the next time it starts running poorly but for the past 2 days it has started and operated like a good ole rover. I guess this would clarify a fuel pump issue or not?
Based upon research an comments, I leaning towards the following:
- Obscure and intermittent fuel pump issue.
- Spotty Coil Pack (need to put a meter on it next time it happens)
- O2 Sensors (unlikely)
- Inertia Switch (very unlikely - http://en.allexperts.com/q/Land-Rover-Repair-1470/reset-Collision-activated-inertia.htm)
- Spotty VSS (probably not)
*latest development : Truck was up to temperature and running well, while wife was driving. She hit a speed bump hard and the truck caught some air. Immediately upon landing it started running poorly again. She pulled over and let the car idle for a bit and the problem dissipated. Something's gotta be loose in this damn thing.

 

You have a loose plug wire. Make sure that they are all connected good and tight on the coil packs or distribitor, whichever you have, and on all the plugs.