Are you sure you didn't hit something metalic laying on the road, and saw sparks coming off that as it moved over the road surface??
There is absolutely nothing that could fail inside your diff, make sparks that you could see, and then still be drivable for more than a few feet. Nothing!!!!!
Believe me - I have rebuilt hundreds of diffs.

That said, if your diff ran out of oil, the ring gear and pinion could be toast, but you would have heard that howling, and there would not have been any sparks.

I really think you need to look back on this whole event, and not be misled by what might have been a hasty conclusion that has led you down the wrong path.
I would also take it to another mechanic for a second opinion.

PK