i have on my focus.

used to get to about 60 and it would start vibrating but it would get worse and worse till about 90 when it tailed off. wheels ballanced, did nothing. new set of wheels (was a prezzy) with new hankook tyres and ballancing spot on still got the vibration.

every time i jacked the car up i couldnt see anything wrong but it drove better for a few min and then got worse again. it would almost go though if holding constant speed on the flat (keeping throttle pedal still) but was worst when accelerating and was bad under braking. thought it could be warped discs or hub.

turned out to be inner tripod joint on the the drivers side shaft. both inner tripod seals were leaking BADLY so replaced both shafts with good ones from the breakers and tha sorted it. was sooo relieved as it took me 6 months to work it out and i had to commute between uni and home a lot, 45miles each way, and meant i couldn't keep up with traffic