1999 Dodge Truck check engine light on and runs rough

The truck is running too rich and fouling your spark plugs, pull one off and see if it is black from soot. Ask your mechanic if he actually put a new O2 sensor on or if he tested the voltage range of what was already on there. Ask him to put a brand new one out of the box on it to clear any doubt. At this point you should check your catalytic converter and EGR valve both function. If the vehicle is running too rich then the cat could be plugged and the EGR could be open when it shouldn't be because of the mixture being constantly below stoichmetric efficiency.