UPR OFF-Road X questions...

Ok making the assumption that the simulator you are talking about is a wire that plugs between the harness and the O2 sensor plug... you need to install all 4 O2 sensors, then install the simulators. If it is something that screws into the hole, then you would put them in place of the rear O2s. Don't run the pipe without the sensors in, you will have a huge exhaust leak and that exhaust will be coming right into the cabin at stoplights.

Just install the o2's like normal untill you put the MIL's in if you need to. They CEL may or may not come on, if it does it will go off when you install the MIL.

I just did this the other day (on my UPR) and it was painless removing and installing the o2's. Make sure you DO NOT cross the sensors, they should be color labeled for the rears, not sure about the fronts. I didnt disconnect the fronts though, just took them out and re-installed them attached to their harness.