2 options there. First is remove the rear cover plate, drain the oil, mop out the remnants, clean and reseal the cover.I suspect mine is low then so will have to sort that out too. Any suggestions how to drain it?
Second option isn't in the book but I have used it on my first Freelander.
When I did it, I drilled a hole in the bottom flat area then threaded it to take an 8 mm socket headed bolt. The aluminium is about 20mm thick at that point and is easy to thread if you have the equipment. Once I'd drilled and threaded the hole I counter drilled a 12mm hole just deap enough for the head of the socket bolt to be flush with the bottom plate. I used a copper washer to make the oil tight seal.
When I changed the diff oil a few weeks back, I removed the rear plate as I wanted to check the diff for wear. Next oil change, I'll drill the casing as above as it's much faster then messing about with the rear cover. Maybe I'll do a "how to" at the same time.