The easiest and fastest way is to drain the pumpkin is to remove the differential cover. Remove the bolts that are around the circumforance of the cover, leaving the top bolt in so that the cover does not fall. gently pry the bottom of the cover and the fluid will pour out. After all the fluid is out remove the last bolt and remove the cover. With some brake clean or solvent, clean the magnet on the inside of the cover. Scrape and clean the mating surfaces with brake clean or solvent, replace a new gasket and bolt up the cover.
Now to fill the differential you must look at the passenger side of the differential housing where it tapers. There is a 1/2" square drive plug, remove it and that is where you fill the differential. Buy the GM gear oil and Friction modifier and a pump for the oil. Pour the friction modifier in then fill with gear oil. Fill until the oil starts to run out of the fill hole or just under. Replace the plug and you're done.