Before you give up on wheels...did you look at them for flat spots? Even if balanced, you can have a flat spot you'd feel. Spin each wheel by hand and look at the rims for a 'hop' indicating flat spot. If you find that up front, move it to the back.
Often times tire balance issues show themselves at a particular speed...and you can often drive up to them and past them without issue...but there will be a range where you'll notice it...and it would vibrate the whole car...so you could think it's coming from anywhere really.
You can see part of the guibo...but if you look closely, see if the metal spacers through the guibo are firmly attached to the rubber. It might look from from the outer edge, but not be where the bolts go through. Take a close picture of it for us.
I'm so depressed. No one wants to come see me in NYC: Sat, Oct. 8, 6 p.m. at Greenwich Village Comedy Club.
Mangofy your E46!