Talking about time frame is simply absurd.
Consider spending at least full day if you have prior experience, air tools & a friend. Or it will take you forever.

There is no gasket. It comes on a silicon from factory.
I wouldn't use black silicon. I would go with either the red (hight temp) or a grey (preffered in this case).

Also, i would change at least the rear seal at the same time (if not the front also). Not sure on the '97, but on '98 & up you have to pull the tranny to be able to take the pan down (by pulling the tranny i mean seoarate it from the engine, not take it off the truck).

And no, there is no way you will be able to do it without removing the diff.

Good luck.