Does it occur at a specific speed or between certain speeds and then go away?
I dont know for sure if the Eibach spring lower or how much if they do, but its likely it could be from the lowering if you have done that. Check that first if you have.
Absent that, I chased a vibration on mine that happened between 45 - 55 or so and then went away, or at least I felt it no more. Three four wheel aligments later, new lower and upper motor mounts, new tranny mount, new tires, etc I found out it was caused by my axle shafts.
