Hey I have a 97 v6 stang. I replaced my air filter with a K&N and I also removed the air silencer, car runs good. I noticed on the highway around 55mph the car runs smooth but when I accelerate to 65mph or faster the car begins vibrating and as soon as I press on the brake and drop down to 55mph it runs smooth. Anyone have any ideas as what could be wrong or how to fix it?

Well, lots of possibilities:
wheel out of balance
front end alignment
bad wheel bearing
drive shaft out of balance
....just to name a few.
Good luck
Dave
Westminster, MD USA
'93 LX 5.0 T5 Ragtop