Our F150 will suddenly start shaking. Usually happens at around 45 mph. Sometimes it is so bad that you have to pull over. It doesn't happen all the time but enough that you can't drive it on major roads for fear of it happening. We have had it to the dealer 4 times and they have realigned it but can't find anything wrong. It took 3 visits until they actually were in it when it happened. It has less than 6000 miles on it. Has anyone else ran into a problem like this?