towing with 4.6
You will mostly likely hear many different opinions on this subject! Many will say a 4.6 should not tow anything! But the reality is that's its all about expectations.
I currently have a 2007 STX ext cab 4x4 with the 4.6, 3.55 greaing and the OEM tire size (255 70 17's). No mods other than I opened up the air intake to a full 3 inches through the fender, which only really has benefit at or close to WOT.
I tow a 23 ft Forest River Bunk house with equializer WD hitch that has a similar weight to yours. I figure around 5500-5600 loaded (at least). There is 5 of us in the Truck, and we don't pack light, the bed is full!
To me the truck does fine! It is what I expected, and its safe! lock out the OD, and let it rip! This is where expecatations, knowledge, and facts come into play! (Look at the torque curvers) It is a power house? No, but it does OK in developing a fair amount of its torgue and a fairly low RPM. Will a larger engine like the 5.4 (Ford) or 5.3 (Chevy) etc, be better at towing....YES!
To summarize, your 2003 is lighter than my 2007, but my 2007 has a bit more HP (230 in yours vs 248 in mine) and the torque curve also changed a bit, but basically equivelent in terms of real world towing power. the 4.6 is a very relieable little engine, and IMO more relieable that the comperable Chevy 4.8. As long as you have Stock tire size, and don't expect pull 75 mph going up long grades, it will be fine! Keep the OD button off when towing, and DON"T get bigger tires, as that lower your final gera ratio which is worse for towing!