Before you begin the drain/fill on any Topkick/Kodiak make sure you have enough gear oil on hand.  You can use any brand you want that meets vehicle specification.  I use Amsoil only because it is easy to obtain.  For a differential oil change you need to have 14L on hand.  If you have a G80 limited slip differential you may want to use a differential additive.  Depending on your oil, it may or may not be needed.  



Start by placing a catch pan under the rear differential.  If you do not have a can/pan capable of holding ~4 gallons you can remove the plug and only drain small amounts at a time.  If you opt for this method keep in mind the oil will drain rapidly should you completely remove the plug.  It is better to unscrew the plug and  hold it in place to slow the drain rate.  This can be done without spilling any oil, but you need to keep shop towels close by to wipe your hands.



The drain plug is the plug located in the bottom of the differential housing in the picture above.  It can be removed with a 1/2 drive socket wrench.



You can expect to find bits of metal on the magnetic plug so do not be alarmed.  Clean the plug and re-insert it.  I like to use a little low temp lock-tite or pipe compound on the plug to assure it does not leak.



With the plug reinstalled all that is left to do is to fill the differential back up.  If you do not have a pump, I find the easiest way is to use a 12" section of hose attached to a bottle.  Simply insert the end of the hose into the fill port and squeeze the bottle until it is empty.  You can expect to use just over 13L of fluid, but you should continue to add oil until it reaches the bottom of the fill hole while the vehicle is on a level surface.