The leak is probably only the bottom plate, I would remove that, drain the fluid and have a look. If rebsealing that doesn't do it then unbolt the steering rack and pull it forward. With the drive shafts out you can remove the circlip said inside the diff and pull the cups / drive flanges out. The diff can then be dropped out.
I would try re-sealing the bottom plate first. I used a squirty gasket from halfords and slowly tightened all the bolts.