2 types of fuel delivery systems out there
- MAF (Ranger's style) calculates the mass of the air coming across the sensor
- speed density, uses many sensors to calculate the proper parameters including its main sensor - the MAP - which is used to calculate LOAD
you say it's missing under load, so let's look there. the PCM is determining load based on the MAF readings (seeing as you dont' have a MAP). it will draw more air under load, and adds fuel accordingly.
MAF sensors tend to hate K&N stuff. the oil required on most K&Ns has a nasty habit of being drawn past the filter by manifold vacuum, and of course the first thing past the filter is the MAF sensor.
Ford's MAF sensor uses a hot wire to determine air density. the wire is heated to a specific temp and that temp is maintained by flowing more current through it as needed. air passing over the wire cools it, so the sensor draws more current to keep it hot. the current required to keep temperature consistent correlates to a mass of air entering the engine.
what happens with the oil from the K&N is that it often coats the very fine current carrying wire. this insulates the wire and throws everything off.
if i were you i would try to clean the MAF with sensor cleaner and run a paper filter. if you have no more miss under load promptly throw your K&N in the garbage and run the factory setup : )
happy hunting. (not saying this is 100% your issue, but 100% it is where i would start troubleshooting. i have replaced many a customer's MAF due to oily air filters)
edit: and yes, try to fix it. a miss of any kind (rich mix, lean mix, no spark or no compression) dumps fuel into the exhaust. great way to plug and blow up cats, creating a much more expensive repair than originally necessary.