Any leak needs to be taken care of but I'm sure you know that.
As far as your axle seal goes it depends on how bad it's leaking. If it's just a small leak right now and hasn't got on to the shoes yet, you should get on it as soon as possible and save your shoes. If it's already saturated the shoes (to any degree), it still needs to be done but the damage has already been done and now the shoes will also have to be changed as well to do the job right, they can't be cleaned, they absorb the oil & you'll have problems down the road if you don't change them....
The other reason this needs to be done is the rear end is losing it's oil and that's the life blood of the bearings, if it gets too low then the problem gets real expensive........