A little extra info on greasy parking brakes I found on Planetisuzu (recommended addition to your favorites!).
If the ventline of the rear differential gets clogged pressure can build in the diff as it heats up while driving.
This pressure can cause fluid to push out past the seal.
In that case clearing the vent and cleaning up the brake parts may fix your problem.
Wiser to be safe and pull the axle to replace the seal but there is no need take it apart any further.
In my case I could feel lateral movement in the bearing so I had to go the whole way.
O, and obviously check your diff. fluid level.