REAR AXEL LEAK

It was dry (could not feel it with my little finger) so I added fluid...then it kicks it out the vent and I hear the axel bearing "humming/grinding" and the diff housing gets real hot. I know its the vent. I replaced it and sealed it (twice). I have been trouble shooting this for a week and have no idea why it is doing it. Any help would be greatly appreciated. What a PITA.