Center driveline carrier bearing
I had a vibration that started at about 110 mph shortly after I bought my 2SS 6M new.
After the miles started clicking on, the vibration would come in at speeds as low as 81 mph.
I had become so used to it that I could tell what speed I was going by the oscillation I was feeling in the chassis. I displayed this "talent" to the dealership service manager after they didn't feel anything the first time.
Anyway, long story short, it was the center driveline carrier bearing, it was shot, they replaced the entire driveline which I was told includes the bearing and no more vibration.
Just my .02
2SS/RS 6M MGW short throw shifter, ADM race cold air intake, Kooks 1 7/8 Headers, Kooks 3 inch exhaust, Pedders rear subframe bushing inserts, Pedders radius arm bushing inserts, Stillen drilled and slotted rotors, "Mike the Powdercoater" yellow powdercoated Brembos, ADV.1 Wheels and Dunlop run flat tires, and VMAX ported throttle body