If the rearend is the Eaton H072 (think salad bowl, or big Ford 9 inch), you can just take the lowermost diff cover bolt out, as the hole goes all the way inside the diff, and let 'er drain that way.
Of course, with not much more work you can do what mugsy said, to check the gears & oil out.
As for the SM420, there should be 2 plugs...a fill (upper) and a drain (lower). Pretty self explanatory
