Had a similar problem once in the rx4, except it happened at 80kmh, gone at 75 or 90, just right on 80, which really sucks when you're on your P's!!
anyway, sent it in for a wheel balance and allignment, which was done and the shake was still there. Guys in the shop said it was casued by the tyres! Because the rx has a little camber, inside were past the tread wears, but the outsides were still legal. This imbalance in the tyre was causing the shake!! Put brand new tyres on her and the shake was gone for good. :evil:
If I were you, I'd deffinenatly be looking at wheel AND TYRE balance first, if possible, buy some new front tyres. If you've still got it after then, then look further abroud. :D