I also have a 93 F-150 4X4, and I occasionaly have/had problems engaging 4 wheel drive. I too, initially thought it was my push-button gone bad, but after talking with some knowledgable folks about it they gave me a couple other things to look at. Seems the connector going to the solenoid that actuates everything (sorry, don't know the exact nomenclature of all these parts) is prone to coming loose, getting dirty, getting wet and basically not making good contact. I took it apart as best I could, used contact cleaner and some dental instruments and cleaned the connector up, put it all back together, used a couple crossed zip ties to hold everything tight and sealed the edges with some RTV. That worked great for awhile, then it happened again, but this time I could hear clicking as I pushed the button, but it never would engage. Had the wife push the button while I listened from underneath (pretty risky considering the amount of life insurance I have on myself) and determined that clicking noise was coming from the solenoid itself. Took it apart and what a dirty mess inside. Cleaned it up real good, put it all back together and everything worked great. Now you may have already exlcuded all this in your troubleshooting...just thought I'd throw it out there for something else to ponder. Good luck!

RWT