I have an 03 Expedition with the 5.4L V8. We have had no problems towing a 24" 2000 KZ Sport. This trailer weighs about 5000 pounds, loaded. I do suggest a weight distributing hitch to keep everything level.
You can safely tow a trailer behind an expedition as long as you do not exceed the manufacturers GVWR, GCVWR, Hitch weight, or the trailer GVWR. To exceed any of these ratings is putting your self and any passengers at risk as well as possibly exposing yourself to serious liability issues should you be involved in an accident. Check the weights and follow the advice some of the folks here have posted.
I tow a Maxlite M28RL with an 04 Expedition with no problems. I use an Equal-i-zer hitch and a Prodigy brake controller. After having other hitches and controllers I'm convinced good stuff makes towing a lot easier.