<font face="Comic Sans MS">To drain the fluid in the rear differential, you undo those 10 bolts that hold the back cover on. First, open the fill plug (mainly to make sure it's not stuck in there, that'd get difficult to refill the differential without it opening) though. [img]/wwwthreads_images/icons/cool.gif[/img] </font>
<font face="Comic Sans MS">The rear cover has a gasket around it. RTV makes an excellent substitute for a gasket. I have used it several times as opposed to a gasket with no problems, leaks, etc. Make sure you get the RTV in a full circle around the bolt holes just to ensure a good seal.
While it might seem easier to syphon the old fluid out, you're still better off pulling the cover, if for no reason other than you can inspect everything in there for chips, wear, breakage, etc... </font>
<font face="Comic Sans MS">In my experiences, the only way to do the front is with a hand vacuum pump and a brake bleeder cup (or at least the most effective way). The front differential doesn't have a cover you can just pull [img]/wwwthreads_images/icons/frown.gif[/img]. Unfortuneately not much thought went into the design of it. Right down to the pathetic 7.2" ring gear. [img]/wwwthreads_images/icons/frown.gif[/img] But I digress. You have to use the pump and brake bleeder cup (or other similar method) to draw the fluid out through the fill hole.</font>