It's most likely the O2 sensor or the Cat...but I think your 04 has 4 cats..
08 FX4... Flowmaster Exhaust, line-X, Sky Jacker leveling kit. 295/65/20 Nitto trails mounted on 20 X 9 Carroll Shelby wheels. AIRAID CAI. Harley headlamps and LED Tails & 3rd brake lights,custom upper and lower Grill. Edge Evolution. Interior/Exterior LED lighting, NFAB step bars, America Tri-fold Tonneau cover.