I have tried pulling 2 different horse trailers with my new F150 FX4 2005. It has a vibration with my old 2 horse trailer at 50-55 mph and with the new 4 horse it occurs at 60-65 mph loaded or empty. There is no vibration at any speed without a trailer. There are several TSB out there for vibrations, but they do not address trailer pulling. I have read a few post of 2004 and 2005 models doing this, but I can't find anyone that found a positive cure. If someone has found a cure, please let me know.