I had to tackle this issue recently on my 128i. I had a small leak from the shift shaft seal that got bigger in recent weeks. I tried to go under there and just stack a second seal on top of the old one but it ended up not stopping my leak.
This time, I ordered a new giubo and the appropriate hardware to take everything apart and do it correctly.
If you're planning on doing this here's what needs to be removed:
1) The plastic under body panels have to come off
2) Rear under body brace (Torx T50 needed)
3) Exhaust midpipe
4) Driveshaft tunnel heat shield (definitely order some new body nuts 07146949380, I needed 3 new ones, I think there were 5 total that I removed to get the heat shield off)
5) Disconnect drive shaft at transmission out put flange, 18mm nut and bolt head (new giubo nuts are recommended) and support the driveshaft. Now is also a good time to inspect or replace the center support bearing.
6) Remove the clips for the shift selector rod and slide it out of the shift lever and rod end joint.
7) Remove rod end joint from transmission shift shaft (This is a pain, use a screw driver to pry the circlip and push it forward. If the sponge in the rod end joint is old it makes it easier to get the pin out, otherwise you need to push forward on the rod end joint and use a pick or something small to push the pin out.
At this point you're ready to remove the old seal.
Through searching for good methods to remove the seal I came across the tool that BMW uses: 83302219637, it was $35 on FCP and worked like a charm, I highly recommend it. To use it: I removed the small screw, pressed the tool hard into the new seal and spun it clockwise, it took a few tries, but once it cut into the seal it pulled in and tightened against the metal of the seal. Then, thread in the small screw and it pulls the old seal right out. The best part is that I was able to use this to get out both old seals, the tool is a small enough diameter that it can get into the transmission recess.
I did measure the depth of the shift shaft bore while I had it all apart. There is essentially enough room in there to stack two shift seals before you run out of room. In my case, the first seal that was the original leak was pretty rough and I don't think it allowed the outer seal to actually seat correctly.