first off ive got a 2000 xlt 4.0 4x4 auto tranny. the truck did great all past winter until late in the seaon i was up in syracuse in a blizzard when my 4x4 quit on me... whats happening is i will put it in 4 high it will sometimes engage sometimes not engage when it engages i would drive a little and not even realize it but i would be in rwd (realize the hard way once i start fish tailing all over the place) same happens in 4low i never use low but figured id try it out to see if the problem is still persisting. ive done some little research around the time of the problem but not too hard figured id wait untill nicer weather to address the problem.. ive read about vaccum lines and what not but dont remember too much.. my question is really how does the 4x4 system work i thought i had a general idea but really i have no clue and also what are something i can try to check for possible problems.. ive got a brake job coming up in a week are two and was hopeing while i was under the rig i can get the 4x4 addressed. any help would be great thanks guys