Yeah, not that hard to do yourself but not having a lift makes the job... less enticing. As far as the CEL, not having a (working) cat. on there could be the reason, I'd wait until you have the new exhaust on there and then clear the code - pull fuse or battery like you mentioned, battery is the more surefire bet.
If it comes up again I'd clear it once more after driving the car a little while before deciding to replace the sensor, and you should be able to tell which one with the scanner. If it's secondary, not primary, I'm not sure if you really need to replace it.