The K&N won't help, the stock air filter/box passes more air than the engine can use anyway.

 

Yea, the K&N is nothing more than a PITA IMO. For pore soles that rely totally on hype. That may hurt yuh.

The 33's make a huge difference over the factory size rubber. They cut into the power more than one may think. For the 4x4's w/tow packages, I believe factory rubber measures out close to 30's.

 

No the FX4 did not automatically come with the 3.73. It automatically came with the 5.4, but not 3.73. 3.55 was stock, but most were ordered with the 3.73. You would also get 4.10 with the 8200lbs GVWR.

 

Kingfish is right, however I don't think you can get the 8,200 package (and thus 4.10's) on a FX4. I'm not positive, but thought I've seen that somewhere. Either way, all of the OP's comments are pretty much in line and make sense for his truck with 3.55's.

 

You stated the trailer weighed 5000lbs. If that is empty weight, I can almost guarantee that it was far heavier when you towed it. What was it's GVWR?
Also have you ever replaced your fuel filter. Should be done every 30k. A partially blocked filter will cause problems.