The 4wd won't engage, it just flashes. How do I fix this so I can have 4wd?

Well first thing is getting the oil leak fixed, if you already know that is a problem. but shifting to 4wd isn't probably caused by the leak. Sounds more like control motor or a switch that is bad without being there to see the problem it would be had to say what your problem really is. But if you get the truck scanned for codes and then post the code numbers, we will probably be able to direct you better.