I've just changed the oil in the front and rear diffs, and the transfer box.
When I pulled the drain plugs for the two diffs, there was quite a lot of sludge (maybe as much as a centimetre high) stuck to the magnet on the inside of the plug. I'm assuming that this is minute bits of metal.
The transfer box one was quite clean.

Does this mean that the front and rear diffs are on their way out?

They have done 136,000 miles, oil didn't seem to have any water mixed with it, it just looked dark brown and old.

Not necessarily as long as there are no other issues such as noise and leaks - just means the oil has not been changed for a while. I would clean the magnets up, change the oil and drive for one full service and then check again 0 if there are lots of swarf maybe look further but I bet it will be OK.

If you have any other issues - well then maybe look further.

Garry 07MY RRS TDV6
Arctic Frost
Aspen Interior

OK, great. I'll keep an eye on them, and maybe change the fluid again in 12 months. Having done it once it's easy.

If the diff oils have not been replaced before, you will have a lot of swarf from when the diffs were first installed when new and they were bedding in. I first changed mine at 80,000 and they were really dirty and as you described. The next change was a little dirty and the rest have been fine.

Garry 07MY RRS TDV6
Arctic Frost
Aspen Interior