Go onto the Ford Power Stroke website and search "vibration at 40- 45
MPH. You will find out post after post on this issue. Ford knows about this issue and does nothing about it.
I had a 2007 PSD 4x4 and had the problem since I took delivery. Five years latter I still had the same problem. Ford engineer and dealer
spent 3 years trying to solve this problem and never did, the went through the motions till they wore me out. That's why my new truck is a Dodge Ram and it purrs like a kitten at 40 - 45 MPH.
What I did due is I found a tire shop in FL that they tru the tires
on the truck which means that they cut the tires perfectly round on the truck and balance them to the truck. That did help and took out about 90% of the vibration. With that I was able to live it until
I finally got rid of the truck. Hope that this helps.(I feel your pain)...