Thanks to all who replied to my original post.
I went and bought the synthetic oil from the dealer because no other auto parts store was open today. I took the front diff cover off and the oil was very dark (almost black). I washed the insides with a cleaner as everything was completely black as well and put everything back together and added the synthetic 75W-140 gear oil.
I tried taking the rear diff cover off but could not manage to get my hands around the top bolts. Its a stock height currently and the gas tank was the biggest culprit. So I went and got a cheap siphon pump to take as much oil out as possible. I think I only managed about one litre (half the total quantity). Again the oil was extremely dirty and had a funny smell of almost like mud and oil mixture. Instead of putting the expensive synthetic oil in now, I topped it up with mineral 80W-90 and the hypoid gear oil additive. I'll run it this way for a thousand kms and change the rear properly next month. This should act like flushing oil.
I test drove for about 20 minutes and made several slow and slightly faster figure eights in an empty parking lot. The feel has completely changed, it is running very quietly and a lot of the vibrations that I thought were normal have gone away as well. I think the previous owner had never had the diff oil changed, even though the services were all done by the dealer.
Thanks TW