Vibration at 45-50
 

i have an 04 f-150 supercrew 4x4 lariat with 103k miles that has a pretty bad vibration between 45-50 after it switches into overdrive, when you turn overdrive off the problem goes away. ive taken it to the dealer and they put shutter guard in it an said to let it work through for a couple hundred miles. it still hasnt gotten any better, does anybody have any idea what it is?

Sounds like a torque convertor having an issue locking/unlocking.

How do you go about fixing something like that?

If it is the TC, you would go about it by dropping the transmission, and replacing the TC.
Depending on your garage set-up, this is either a simple to mild task..... or a massive PITA.