Trying to get some ideas about this vibration issue I am having. Just got a 2008 F150 FX4 with 82k miles. Issue is after about 40mph it's starts to vibrate pretty bad. Doest matter if you slow down or speed up. Continues to vibrate. Will stop around 35mph. Seems to want to vibrate when it shifts or down shifts the most. Took out rear drive shaft and vibration stops. Replaced drive shaft with a known good on, still vibrates. So basically ruling out ujoints,drive shaft,brakes,tires,or front end actuators,spark plugs and coils. Thought maybe rear end clutch since it's a LS. Replaced fluid,old fluid looked horrible. Still vibrates. Anyway if you have any ideas shoot them my way. Thanks in advance.

 

Are the tires a matched set?
When you replaced the fluid did you also add friction modifier?