Don't know what size Ford you have, but many later F250/350 4x4 models have a rear axle bump/vibration/etc at 40 to 50 mph. I had a minor one with the original Continentals at 48-50mph on my F350. With the new Kumho's it's even worse and they have been balanced twice on different machines. The vibration is at 49-52 now - whoopee, it moved up a mph. Supposedly a Hunter Road Force balance machine will fix it, but not always plus it's $25-40 a tire if you can even find a place to do them. Go to Ford Truck Enthusiasts Forums - Ford truck and SUV owners and enthusiasts Community And Information Source. and plug it in and you should have many search hits. Welcome to the world of fanny massaging via FoMoCo