Funny...I'm having the EXACT same problem. I have had my alignment and wheel balance checked by 3 different shops. I'm thinking I might go to a fourth. Only mine shakes horribly between 85-90, 120 and up it's smooth.

I noticed negitive camber wear on my rear tires, mabey this negitive camber is causing it, I'm going to space my camber kits out some more (even though according to the alignment shops it's aligned, the tire wear says otherwise) and see if it goes away.

I also think I have my rear slotted discs on the wrong way (instructions didn't say which way they should point). Mabey this could be causing it?