2003 F-250 Vibrtation

At work we bought a 2003 F-250 at an online auction. I was driving it home, and when it shifts into fourth gear, (overdrive) a vibration develops. When I pushed the overdrive lockout on the gear selector and it downshifts to 3rd the vibration pretty much goes away. Turn OD back on and it upshifts and the vibration is back. The transmission seems to shift through all the gears really good / smoothly and is glass smooth when you go from Reverse to Drive. What could this be; fourth gear going out, sticking valve in the tranny, drive shaft off balance? Just looking for some ideas, not sure where to start on this one. Thanks in advance!

I would replace u joints if that don't work u might need a new drive shaft