1998 Ford Explorer O2 sensor codes

I have owned my Explorer since 2002, sometime in 2003 I had the exhaust replace from the cat back including a new higher flow muffler. The new exhaust is slightly larger than stock. Soon after that I started getting bad o2 sensor codes and noticed a steep decline in fuel milage. Since then up until now I have replaced all four o2 sensors several times but it doesnt seem to do the trick. I have had this into several dealer shops and independent shops and nobody can figure out what is wrong with it. I can clear the codes and they come back as soon as it gets through a warm up cycle, but if I just drive it as normal and ignore the check engine light long enough (few months) I start getting misfire codes as well, but will stop if I clear the code memory. The constant codes I get are: 0153 o2 circuit slow response bank 2 sensor 1, 0174 bank 2 lean, and 1151 lack of Ho2S21 switches - sensor indicates lean. Now also within the last few months my car has been intermitently losing power, not sure if this is related or just another problem being caused by whatever the first problem is, but it is all really starting to annoy me and I want to get it fixed once and for all. I am assuming that this has something to do with the exhaust change out, and I'm hoping this can be fixed without putting stock size exhaust back on. Thanks for any help.