Check the mounting of the wheel to the hub. 40-45mph shakes have happened to me on 3 cars and all were to do with not having a perfect wheel-hub mount.

I had it a few days ago on my X5 after getting my ARB links changed. Never had it before - it made no sense that it could be ARB links causing it so I took off the front wheels. I noticed the drivers side wheel was difficult to remove from the hub after the bolts where out, also the bolts seemed to be tight at a certain point on each rotation when I was undoing them suggesting they were in at a slight angle or the wheel was at a slight angle exerting weight on one point of the bolts.

Anyway, a visual inspection of the hub (after wire brushing all the old grease off) showed that the wheel centering flange of my hub (the bit that goes inside the wheel and the wheel centers on) had a slight lip at one point on its circumference.

I noticed the inside of my wheel had a matching 'cut out' where the 'lip' would sit comfortably. Once put the wheel back so that the lip sat in the cut out, it sat on the hub nicely and the wheel bolts went back in smoothly. 40-45mph vibration gone.

I have no idea how my hub & wheel developed these matching defects, but as long the wheel is mounted in the same way each time it drives fine. Obviously if I ever get new wheels I will need to get a new hub or get it machined.

Anyway, my point is check your mating faces, see if there is any thing putting the wheel at any sort of angle on the hub instead of flat. I read a forum post on an e39 forum of a guy who had this problem until he just kept changing which bolt hole in the wheel was mounted over a particular bolt hole in the hub.