upgrades to improve towing in a 2001 Expedition?

Having towed with a 2001 Expy for years (with 3.73) I can tell you that the weak link is the transmission. I would do nothing more than maybe spring for the 3.73. If you start messing with tuners, headers, etc, you will soon be in the shop for transmission work.

As they like to say, "It is what it is." Stay with a trailer around 5K dry, skip all the bolt on band-aids, and save your money for the kid's college.