Ive changed them on dirt bikes and it was no big deal, really. I believe the seal was less than 10.00.
The impact gun on, countershaft sprockets, is not my favorite tool. Then again, I'm sure you were screwed, just like I was. I can almost guarantee Andre The Giant tightened mine, at the factory, before the bike was sold. Then he did the oil filter....
Check that shaft for excessive play. Its not a real common thing to have leaking and it makes me wonder if something was damaged when you used the impact gun.
All I did, for mine, some seal lube, then slid the new one in place.