As mentioned above the two main causes for axle seals leaking are a blocked breather or a bent housing...
However...
I have seen quite a few newly installed axle seals leak due to lack of care when refitting the axles... The seals are EASILY damaged by sliding the axle on them.. Be very careful not to touch the axle against the seal when reinserting it.... Pre-greasing the seal helps too.
On a side note:
If you fit mini backing plates you can change an axle without touching your brakes. No mess.. No rebleeding..
These are available from
www.lowrangeoffroad.com for US$8 each or can be made at home out of a spare pair of backing plates.
Jas.