More than likely something wrong with one or more of the things you replaced, could have cracked one of the plugs when putting it in, coil pack could be bad etc... are the plug wires in the right place?

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2000 XLT 4x4 EXT. CAB 4.0/Auto Trans/3.73 gears/
31x10.50 BFG Rugged Terrains on MB Motoring Wheels/Pioneer Head Unit/JBL 12"/Camo Interior/'03 black and grey 60/40 seats/ green LED's Under Dash/T-Bar Crank/Windows-20% on the front, 5% on the back/Mustang Dome Light/06 Tails/