How do you change the 4x4 flluid on a 99 silverado

You should have two plugs on the back of the unit (in most cases)
One is higher than the other.(usually they are about 1" in size) High one is fill...remove that one first (if you can't remove it then you can't refill it, so don't remove the lower one) Once fill is free, remove the drain, wait till fluid stops running then replace drain plug and fill till fluid just seeps from the lower edge of the hole (on level ground) Replace the fill nut and you are done!