So I've noticed this winter I have a wicked vibration coming from the front end. You can feel in the the steering wheel and in the seats. The whole truck shakes. It's only when I am traveling 60-80 km/h (37 - 50 mph) and my rpm sits under 2000. We've replaced the brakes(front and back within a year), alternator(last week), and valve cover(a year ago). Everything underneath looks all good. The vibration is worse in 4x4 as the roads up here are horrible in winter. If it's a dry road I put it in 2 wheel to see if the vibration is still there and it is but not as bad. Any ideas what this could be? It's a 1999 f250 Super duty 5.4L.