Engine or Transmission Power? Please help!
 

I purchased a 94' F-150 5.0 Litre truck a few months back. I drove it for 3 months with no problems. I shipped the truck to Puerto Rico and have experienced problems of no power going up steep hills. I have to use the 4wd low gear to be able to summit some of the winding hilly roads. On level ground the truck seems fine. Any clues as to what may be wrong? I changed the plug and wires and air filter.

I have tried putting the steering column in both drive and low gear and the 4WD transer case in low gear and sometimes the gas pedal is all the way to the floor and still hardly no power.