Started having a vibration at 20 to 30 mile per hour, IT only does this when transmission is in overdrive, if you bump it down to 4th no vibration or if you accelerate to 20 to 50 in overdrive no vibration. Dealer said it was transmission, Also I bought the truck new and not always but sometime I will be stopped and it feels like someone bumped me in the rear end I have had it to dealer many times for this and they say they don't know what it is.