Hey everyone! Firstly let me thank everyone here who reads this. I'm going to provide everything I can and might be a bit lengthy. Firstly, loving the expedition. I bought it to move my family and all of our stuff over 1500 miles with a trailer weighing just over 10k. It did amazing. I did all sorts of tune ups like the achy breaky spark plugs, wires, PCV, cleaned carb, belts, all the fluids, etc. I did not replace the fuel filter, about the only "maintenance" item I didn't touch. O2 Sensors seemed a bit touchy as well, but left them. So we get to where we were going and I start having issues towing the same trailer empty (around 3700 empty). It did ok at low speeds but became questionable at times if it was accelerating at all past 40 MPH. With that issue I also noticed a slight humming/vibration between the transmission and the rear of the truck (atleast it sounds that way, the sound could be throwing itself). However, she still made it to where I needed to go. Just slowly. Overdrive on or off (which I also keep it off towing) didn't seem to make a difference. So I sold the trailer, and about a month later just the truck itself is having issues accelerating around the 50 MPH + marker. However this time I can tell the humming/vibration has gotten worse and seem to isolate the issue being problematic around the 3rd gear. Keep in mind, I can still, very barely, notice it all the time. It just gets real prominent when faster/higher loads like wind is pushing on the car, specifically when I downshift from 4th to 3rd. My thoughts are the plates in the diff, or maybe something like that. I've heard some say crank sensor or EGR valve (which I did not clean, prob should of) There is also a exhaust smell of unburnt fuel that sticks with the car just about everywhere. Might be unrelated but I've tried cleaning IMAF sensors (with proper cleaner) and checked all vacuum lines. I'm hoping cleaning the EGR valve and replacing the fuel filter this weekend will fix both problems, but I doubt it. What do you guys think? I know this is a lot but we all know how complicated a problem can get. Thanks for any time and effort, I'll gladly answer any questions. 2006 Ford Expedition, 170k miles, 2 WD, Towing package (has the right towing diff). Average MPG between mostly highway (i do about 75/80 mph) and some city is 14.6. Hope we get some answers!