Sound like little explosions in the exhaust

[Steven419]

for starters i am working on a 1997 Ford Explorer 4.0L with sohc, runs and drives good except when driving down the free way you can feel a hesitation and like a backfire in the exhaust. changed the plugs and wires, primary ignition is good, have good short term fuel trim about-3 and long term fuel trim at -4 on both banks. any suggestions would help