Not sure how best to describe this. I have a 2001 F150 with less than 70K on it, runs fine mostly. For the past few months while driving at speed, say 60 or so, it will start vibrating as if it's running on something less than 6 cylinders. I can see the steering wheel shaking and it's hard to tell if it's the engine or something else. I watched the tach today when it happened and it was spot on at round 2K RPM, didn't move one bit, so I'm not sure if this means it isn't the engine. I've never heard of out of balance wheels cause this intermittently. I would think if it was that, it would always act the same way. Then if I slow down to 40 or so, it seems to go away and I can get up to speed again with no problem.
Any ideas would be appreciated.
thanks