can be done with g/box in place look on brokevw.com for how to do it, you need a bit of 10mm threaded bar and nuts, 8mm thread long bolts and nuts, a 3 way harmonic gear puller, circlip pliers, and 22mm ID tube .
whip off the drive shaft, remove centre cap from flange, remove circlip holding on flange, using puller with the 8mm bolts pull off flange [it may pop off under spring pressure] .
then fit new seal making sure it sits straight [ its worth setting new seal about 2mm further out from original position so its not on wear point of flange]
then using 10mm threaded bar screwed into centre of spindle in g/box wind flange back on using puller, use nuts on threaded bar to wind back in, put circlip back on using the tube.
its explained on brokevw in much more detail with pics under drive flange removal,
as clipperjay said your seal is farked mate