Drop in a K & N air filter it will improve your torque. I have the same van as you do and pull a 21 ft ROO trailer. the 5.4 will tow it 70+ Mph in overdrive anywhere except steep mountains and get 11-12 MPG in overdrive. out of overdrive you will get 5mpg. Durability ? I have a friend that pulls a 29ft Trail Lite, he has over 250,000 on his van no problems. Watch when you drive pulling the 5.4s have a sweetspot around 72mph that the torque is unbelieveable. 3.73 will give you more power and 1 to 2 less mpg | 
