Hi! I have a 2009 sts with you 105,000 miles. It is rwd with the northstar. I’m having a vibration issue at around 75+ mph. It is in the steering and the seats. It seems to not be as noticeable on rough roads as it does on smooth smooth roads. I have had the tires replaced and had them balanced multiple times. I have also had the car aligned multiple times. I have had all of the suspension checked. I have also replaced motor mounts, transmission mounts, carrier bushing, and rear differential...
2009 STS highway vibration
2009 STS highway vibration