2001 Ford F-150 jerking at 45-55 speed

Not sure what category took to repair shop said it maybe coil or spark plug or wiring he put it on his anaylsis it show nothing amiss in engine he said he fixed a wire but when we drove it still have the issue I thought it was the transmissiom but know one knows I get all different answers I purchased it last July and drove to I to Utah and back with truck bed filled with furniter and in Utah had to put sand baggs for snow 400lbs at 45-55 it starts to jerk alittle then I gas it to 60 it stops?