You probably have one or more tires out of balance.
Most likely, one or more of your wheels are out of balance ... they often smooth out at higher speeds. Another likely hood is that one of the tires has its cord bent ... take it to a tire shop.
Thank you NDWind.
Lack of oil in engine, transmission; wheel bearings, driveline shafts, tires, !wheel balance, misfiring, computer mapping glitch, fuel delivery inconsistency, air intake problem, exhaust change problem. Most problems present in a circumstance first and develop into all the time eventually. If it doesn't develop into something worse it may be costly to find.
wrong oil, incorrect pressure air in tires, wrong exhaust, wrong tires, wrong balance, loose tire nuts, incorrectly adjusted wheel bearings.
you make it sound so simple, thank you for the great advise...