Anyone else experiencing vibration on the back end of their 2004 F150 SuperCab? Right at 55mph it vibrates heavily, and smoothes out at 60-65mph. I've heard this is a common characteristic of the '04 design, but how can I fix this vibration once and for all. If you look out at your side rearview mirror you can visibly see the bed wobbling....Let me know if you have nay suggestions before my truck flies away like a mexican jumping bean......