Re: diff cover
The front diff doesn't use a cover. So you don't have an easy choice there. 2nd generation S10's have a drain plug. But that means you've got to swap the entire front diff (case, gears, diff, everything). Not an easy answer. I've seen a thread on here about drilling ones own drain plug hole, but I'm not sure if that's a good idea.
The rear diff should be easy however. Check Summit & Jegs (or your favorite vendor)....They've got plenty of choices for 7.5" / 7. 625" 10 bolts.
The easiest solution, however is to just add 2 drainplugs which are available as kits (no welding) or you can just drill some holes / weld on some nuts and use drainplugs w/ washers from somebody like AutoZone) to your existing rear cover.
'JustDreamin'