My truck recently started to misfire or backfire while giving it gas in gear while going down the highway, can anyone give me an idea?

start with the easiest and cheapest first.cap,rotor,wires,plugs,around $55.00..had one a while back,diff vehicle,would not hardly run at all.they wanted to change iac valve,tps sensor,several more hi dollar items.i said no,not just yet.pulled the plugs out,they were like fused/welded together?come to find out,those plugs had been in there a VERY long time,like 150,000 miles?..put new plugs in it,they drove off in the wild blue yunder,running like new!