I'm trying to figure out the vibration issue on my truck. I have an 04 f-150 xlt, 4.6L with 31,500 miles. At around 6000 miles i had this issue come up and found a defective rim that was replaced under warranty, then tires road forced balanced and was good till about 28,000 miles. I've had two alignments and it's been out on the right side both times. The vibration starts at 40mph at times but is most noticable at 50-55mph. sometimes it's bad enough that the bed of the truck bounces. Any thoughts?

 

Don't know but if you figure it out, let me know. My 2006 XLT 5.4L 4x4 has the same problem.