Hmmm...I would guess either brake rotors or wheel hubs if you only feel it at speed. Check all your brakes and see if it looks like the pads are grabbing or not. You might have to pull the calipers off to see the back pad as that is the one that usually grabs(Murphy's law).
My mechanic diagnosed a bad wheel hub for me and he was amazed that I couldn't feel any vibration at speed. The hub is not too hard to replace(if that's what it is). The ABS plugged right up to it and not computer reset was needed.
Do you have wheel weights on all your tires?