Where are the oxygen sensors located, and what tools do I need to replace them

They make a slotted socket specifically for o2 sensor removal but you can try removal with 7/8 closed or open end wrench. Main issue : They can be really jammed in there and require a lot of torque to remove. Pre-soak with PB blaster to try to help break free. The slotted socket usually fits a 3/8 socket wrench. Location can make removal difficult as well. Good luck : http://www.autozone.com/repairguides/Ford-Excursion-Expedition-Lincoln-Navigator-2000-2005/Component-Locations/Heated-Oxygen-Sensor/_/P-0996b43f802e7449