If you want an indestructible diff cover, I would go with one of these. I think they only have them for the 8.25" rear, though.

As for cleaning out the diff, I usually let it drain for a while (maybe a couple of hours) to get as much as I can out of it, then just wipe out the bottom of the housing with a rag. There's really no benefit to flushing it with clean oil or any kind of solvent.

75W-140 oil will work fine. If you've been using it and it's working for you, there's no reason to switch.

If you see any kind of springs in the center of your carrier, it's a limited-slip.

Finally, RTV silicone is a perfectly acceptable way to seal the cover, though I prefer a gasket (Lube Locker is the best, but Fel-Pro is good) installed dry. It makes it much easier to remove later and with the heavy-duty cover, it seals very well.

 

With the vehicle on the hoist, give one wheel a spin. If its a limited slip, then both wheels will turn the same direction. If its an open dif then one will spin the opposite direction. All this is assuming that the truck is in Park so the driveshaft cannot turn....