[lupinsea]

One thing that a lot of the Jeep guys do before they wrench on their exhaust is hit any threaded connections affected with PB Blaster or Liquid Wrench the night before. I've done this a few times and it's surprising how well the products work.

Also, would it have been easier to remove the section of exhaust the O2 sensor was on so you would have had easier access to the sensor with the header pipes off of the rig?

Of course, you're done with it now so it doesn't really matter that much.