Alright, I think I know where I am at with this but the input from the members on this site is always helpful. I first noticed vibration from the rear of the truck when turning left or right while towing my travel trailer from a stand still or slow roll. Now i am starting to notice it more often even when I have no load at all. I believe that is has to do with the limited slip rear. I was told just change the rear fluid with a synthetic and 4 oz friction addative from ford but I read somewhere that some used mobile one and didnt need the addative. I want to do it right once what do ya all think.