I had a vibration from the rear of the car that appeared by 70 mph, by 85 it was very Noticeable. All 4 wheels had been recently re shod and ballanced, however when I replaced the 3:27 diff with the 2:87 one I noticed that one of the rear wheels had a slight buckle in it. I moved the rear wheels to the front and visa versa and hey presto she'ssmoothe as silk upto 174mph (tested on a airfield). This follows the old advice I have been told before that prop vibration will appear in a certain speed range where as an increasing vibration with speed is normally due to either disc/drums out of balance or wheels. Turns out to be spot on.