That sucks. Your now messing with the crush sleave and pre-load settings. I have never heard of a leak between the splines before. As Tohcan said you could coat the splines with sealant and try putting it back together. You might get lucky and get the nut tight enough to stay on and not crush the crush sleave any further.

If it were me and it has been before. I would pull the rear end and take it to your local rear end specialist and have a new seal and crush sleave put in. You have to know your limitations and rear end work is mine. Not that expensive and gives peace of mind.