There's a thread on TDS that discusses this and of all the responses this one was unique. It's the third entry down by fullservice. If the link doesn't work it's www.TheDieselStop.com - 1999-Up Super Duties - 6.4L Powerstroke and Drivetrain - Vibration at 10MPH when towing
http://forums.thedieselstop.com/ubbthreads...e=0#Post3367278
I'm no expert so I make no statement about the rightness or wrongness, just sharing another perspective.
Steiny