If you can't borrow a 30mm tube socket, you may have to make one. You will need a standard 30mm socket (preferably 3/4 inch drive as this will allow it to fit over pinion nose), small length of water pipe and a length of mild steel bar.
You will also need access to a welder.
First grind the chrome off the back of the socket, next weld the pipe to the back of the socket, then weld the bar to the pipe (longer the better for leverage).
Now to replace the seal. First mark the nut to the pinion nose with with a dob of paint (this way you know exactly where it was positioned) as long as you don't go past this mark you won't have any problems.
Now you will need to hold the pinion yoke, you can use a length of bar with two bolts welded to it so they fit through two holes in the yoke, then crack the nut loose and remove the yoke (you may have to lightly tap the yoke to remove).
Now remove and replace seal, clean up yoke, lightly oil and refit.
Refit nut and tighten back up to where you have marked it and your'e done.