Dean,
Not sure I agree with Jason advice.....really a pretty poor tow vehicle (and this is coming from a guy that towed a Sprite on an un-braked open trailer with oodles of spares first with a short wheelbase Aerostar and then later the same stuff with a long wheelbase Aerostar...boy do I have stories).....almost entirely associated with the short wheelbase.
Later I towed a 20' enclosed trailer with lots of spares with a 98 Expedtion and at times the silly thing would get to swaying back there in a funky and most alarming manner. I really never towed faster than 70, usually 65 cause above this speed I felt that I no longer had much control of the silly thing. Although I had a nice load distributing hitch a friction sway control device did seem to handle 50+% of the problem.
My opinion....anything over a 20' trailer with a good hitch and sway control.......you're really asking for trouble when the winds pick up or a semi passes you. I also had the 5.4L Expedition engine that was just BARELY adequate power wise....but you can work around that shortcoming. No working around the wheelbase issue. Of course the mileage sucked but no surprises there.
Now have a 2004 F150 PU with the Triton 3 valve engine (beware of spark plug issues) and it tows like a dream (20', SM, reasonable spares) without much more power than the Expedition. Night and Day.