It's not the flange that is leaking it is the seal. You can't RTV a seal on a moving shaft. The previous owner replaced the seal once and it has started leaking again shortly there after.
The hack and tap basically pulls of the slip yoke and makes you cut off the shaft. It then places a seal in that flange on a section the shaft that never had a seal. The advanced adapters kit not only removes the slip yoke but also the speedo gear housing with a new housing with a seal in it and you also put in a new output shaft.
The difference is the hack and tap you can do it with everything still installed on the jeep. Other brands you have to pull the transfer case and take it apart.
I would prefer to pull it apart and be done with it if it is the better option then replacing a seal that will leak again shortly down the road.