Braking system on my 2008 chevrolet silverado vibrates harshly at speeds above 50 mph. the higher the speed, the more the vibration i feel, making it hard to stop and making the vehicle feel unstable. there is also a popping noise that has been coming from my rear brakes. these issues started at about 5-6k miles and have continued to get worse. i now have 24k miles on my truck and gm has not been able to fix these issues. *tr