Let's hope someone who has done a 99 Legacy pops in with more info, but if not, here goes. I've done lots of half shafts on Subys, but not a 99.
No special tools needed. Top of my head I think axle nut is about 150 Ft Lbs. Its easy. Undo axle nut, jack up remove wheel, all that normal stuff.
I always have to separate the tie rod end from the steering knuckle. Off the top of my head I am thinking I had to undo ball joint on some models but not all. If you do, no biggie. Very easy.
By the way, I think it is a waste to try to rebuild a half shaft yourself, just get a rebuilt unit from NAPA or other name brand. If you need a new axle nut, it should come with it. Otherwise, it probably has a cotterpin type.
Knock out the tension pin holding the axle to the transmission output shaft near the transmission. Save pin unless you are (recommended, $5) replacing it.
Pull the steering knuckle far enough out away from the engine to remove the splined end of the axle. Turning wheels can help. Press center of axle out if stuck (any big puller) or put axle nut back on axle and tap lightly with a huge hammer (if stuck) untill it loosens up. Then slide out.
Good time to turn wheel bearings by hand after axle is out and check seals. Some axles have an interferance nut which has to be punched with a hammer instead of having a cotter pin.
Reverse to install.
The Ole Rep (not Subaru)