4X4 leaking?????

[01lariat4x4]

i have an 01 f250 4x4 with 86000 miles. last night was the first time the i used the 4x4 since i have bought the truck. to my suprise when i arrived at my destination i noticed a trail of fluid behind me. i was leaking tranny fluid the whole time! my tranny has never leaked before and now that i was running it in 4x4 it was. it the leak looks like its coming from the back of the transfer case when the little guard/steel plate is at. and it was leaking bad enough that it was shooting all the way to my passanger side front leaf spring. however once i put it back in 2wd it stopped leaking completely. any input? same experience? i am a noob to 4x4 so any advice would be greatly appreciated.