1999 Ford Explorer leaking transmission fluid and 4WD low

Hi, I have a 1999 ford explorer eddie bauer edition with 165000km. Just a few days ago I found a small puddle of transmission fluid in my drive way and then check the dipstick. It was almost empty, so I looked under the truck and I could see a lot of oil all over the place. My transmission still shifts fine, theres no 'clunk' when shifting gears but what I did notice is there is a 3 position 4wd switch to the right of the dashboard. I swiched it from auto to 4wd hi, and the light came on. But when I switched it to 4wd low, it wouldnt switch from 4wd hi, and there was no light. I topped up my transmission fluid thinking this was the problem but it still wont switch to 4wd low. Im just wondering if this is common on these trucks, and what could the problem be. If you could help me out it would be appreciated. THANKS!