Downstream O2 Sensor?

I have never replaced the after-cat O2 Sensor on my '96 XJ and have replaced the front O2 Sensor 2 times over the past several years. I have 225K miles on the XJ and have started getting the P0138 codes again. I thought I'd fixed it before w/out replacing the O2 sensor.

My question/concern mainly is the fuel mileage. My mileage seems to be getting worse by the week. Everything that can effect mileage has been checked, tested or replaced (other than the rear O2 Sensor) and yet my mileage is still around the 13-14mpg (highway & city driving) whereas I have averaged about 18-20mpg (highway & city driving) since I bought this XJ about 6 years back. No mods have been done to the XJ that would effect mileage in that time.

I really don't want to have to drop $60 on a new O2 Sensor if it really has nothing to do w/performance of my XJ. I suppose my Cat could be going bad as that's not been replaced in over 120K miles.... but I don't want to just throw money and parts at the jeep unless it's money well spent.

Any gains to be had from replacing the rear O2 Sensor? Anything like replacing the front O2 Sensor?

Thanks

How long since you cleaned your injectors out with a good FUEL SYSTEM cleaner. I do ours every 6 month, 20oz bottle of Chevron Techron FUEL SYSTEM cleaner, not injector cleaner, the fuel system cleaner has twice the solvents that the injector cleaner has. My 98XJ's cat and O2 sensors are original and that XJ has close to 400,000mi on it, it got a jasper reman engine around 330,000mi and new Mustang injectors from Five O motorsports.

I have a 96 and I'm still not sure about that 2nd o2 sensor--it's supposed to check up on the 1st sensor and throw a code for overly rich/lean conditions(or for bad heater circuit). When I swapped the Borla exhaust from the 94 over and used the magnaflow cat w/o a bung, I tried running with the sensor zipped to the frame in both connected and un config. Unfortunately, I was fooling with the Hypertech 3 programmer at the same time-which did nothing for my XJ except to clear the codes that it caused in the first place. It was running rich. Later down the road I bought a 2-bung mag that I accidentally melted during psc tuning(you have to fiddle before you get it right--cat doesn't forgive) and so now it's hollow and the 2nd sensor still thinks all is fine. In your shoes, I would spend $60 on a hi-flo cat before that 2nd sensor.

I have run 2 bottles of fuel system cleaner over the past 6 months and also checked my fuel pressure and that all seems good to with-in spec. I'm wondering if I missed a bad/melted/busted wire coming f/the rear O2 Sensor.