er......... put it this way, the outer case protects the diff inside. now, the diff is actually sealed by the main gear sandwiching a rubber gasket between it and the inner diff case.
Your not looking at "much trouble" if you take your time. If your this far in on the strip down, it makes more sense to check the inner diff gears NOW, rather than have to do it later and tear half the truck to pieces again. Diffs have a habit of running dry if they have had oil used in place of the factory grease. Since you don't know the heritage of the truck too well, you don't really know if oil was used ( this is an assumption, so correct me if i am wrong here). Running a diff dry, wears the gears out a LOT faster. Also, diff oils can slow the action of the diff and reduce "unloading" from the wheel that is on the ground on one side to the wheel in the air on the other. some call it "limited slip" when this is done but in truth, it simply slows the diff gears down the thicker the oil gets. I.E 10 will alow more slip than 50K oil.
Trouble ?? well, you need to inspect the teeth of the gears for wear, the bearings for wear or being dry (a dry bearing sounds noisy when spun and spin a lot longer than a fully lubricated new one). Things to look out for when checking bearings is if they feel "Gritty", a sure sign of a problem. If you find a dry bearing, try lubricating it and see if it rolls smooth and plush. if in doubt, swith it out. bearings are cheap, tearing a truck down to repair parts that would otherwise have been ok is heartbreaking and money that could have been avoided
The actual outer housing is not "sealed" as sealed goes. dirt CAN and WILL get into it. Especially water. and water can carry mud and other stuff in aswell.
My advice to that problem, would be to get everything cleaned and use some clear silicone sealant on the meeting faces of the housing and see if you can get a rubber o-ring to fit over the output shaft and behind the output drive cup to help avoid getting water/muck etc in there again.
I kicked myself for weeks after i ruined a set of gears in a buggy diff. i had the whole thing stripped out with the diff in front of me. never did check it. put it all back together after i finished cleaning and the back end started getting noisy. turns out the diff ran dry and the gears wore. a fix i need not have done had i have checked it in the first place.
Do not get me wrong, i am not trying to worry you at all. but opening that diff just to check all is well could save you time and money in the long run.
Oh well..