I saw this in the news and thought I should pass it on. It seems there is an issue in 2011 and 2012 model year F150, expedition, mustang and Navigators for faulty speed sensors. This is causing the transmission to suddenly shift to 1st gear. Probably not a good thing towing a large, heavy trailer. Here's a link to what I read.
Ford Recalls 202K Pickups, SUVs, Cars to Fix Transmissions - NBC News
Ford Recalls 202K Pickups, SUVs, Cars to Fix Transmissions - NBC News