I found a couple old threads on this, but didn't ever see anything conclusive on how to fix the issue. When I reach about 75 mph, the rear passenger seat shakes almost violently. I can see the headrest vibrating in my rear view mirror. I've checked all the accessible bolts to ensure they're tight. However, I can feel the whole car shake a little at that speed as well, but it's most visible and manifest in the back passenger seat. Perhaps based on how it's fastened compared to the other seats? I had the tires balanced, but it didn't help the issue. I owned a 2009 RAV4 before I purchased this 2012, and it didn't have this problem. I appreciate any help!