Autodoc sell these genuine ZF kits at a reasonable price.  You can find the procedure to do the job yourself on YouTube.  It doesn't look to difficult to do if you're an half decent diy enthusiast. I'm going to my sons BMW 335d which has the same gearbox. 

Good idea. Try it out on someone else's car first.😁

There's a great video on YouTube where this guy does , his diffs and gearbox .  The front diff took some getting at mind.   Was on a sq5 but I'd assume it's a similar process.  His was on 100,000 miles . The diff oils and Tf looked burnt that came out.