For towing, I'd look at the engines, transmissions, gear ratios as these are the expensive parts to change. I'd suspect the Expedition to have either a 4.6 or 5.4 and the 4r100 which would be pretty decent if you have stock tires. The 5.4 with a trans cooler plus 4.10 gears would be the way to go over say a V6 with 3.08 gears in a bronco. The I6 or V8 bronco would be a start but I'd look at the tire size and gear ratio.