At 40-45 mph this vehicle may exhibit such a jerk when it is time for a tune up. At this cruising speed egr flow is present and can overwhelm a weak ignition system causing such a symptom. Don't skimp on parts. Do a major tune up. Check the fuel filler neck for cracks or excessive rust. Take off the gas cap and check in the filler neck. Also check gasket on gas cap for cracks.