BrandKoe, very nice information:bigthumbsup
OP, like he said^^ I doubt its your clutch(err...hope its not at least)...But my guess would be that your wheels are imbalanced. I used to work at discount tire for a couple years and I would hear this all the time, "but, but , but sir...it only vibrates at this certain speed, etc."(not making fun of you, just all the people who would come in w/ this prob)
I would take it to discount or your favorite tire shop, and have them zero out all of the tires. Make sure you go into the garage or can at least see the guys balance your tires off, b/c some times...err a lot of times they will get "close" and just throw them on.. So imo, I would do that first, b/c it is cheap and could probably use it anyways; and than go from there. But I really do think that it is something besides your clutch, either way, good luck:bigthumbsup