About six months ago I acquired a 300tdi Discovery to replace the Series III that had given many years of faithful service, so I am new here but not to the world of Landrover "fun"

After a couple of months it started struggling/rumbling/vibrating above 70, slowing getting worse as I tried to track down the cause, not fuel, not propshaft UJs . . . Anyway it turned out that the big nut on the back of the transfer box was only finger tight, so I took it off, cleaned it up and put it back together and torqued it up. Great, vibration much better.

However, it soon started up again so I bought a flange/seal kit and cleaned it all up and replaced everything, there was no obvious play in the output shaft and the splines looked fine. It must be sorted this time!

But now it has happened for a third time, this time coming apart enough for the handbrake drum to move backwards 1/2" or so and dump most of the oil into the handbrake . . . .

I am loath to repeat the same mistake for a forth time without knowing why it is happening, so am asking if anyone has any ideas as to what it going on, as an internet search for this problem has turned up nothing.

How can it come undone? Doesn't the rotation direction of the output shaft act to tighten everything up?

A local indie Landrover garage suggested using a genuine LR kit and some super strength loctite 271 on the nut. Is that it or is something knackered inside the transfer box?
Thanks.

It should have a nylock nut on it? I only had 1 come loose & that was in the racer so was shook to death. Lucky I had disk handbrake so was simple job.
Have you got any play in UJ to cause vibration which then loosens the nut?

Put some heavy loctitie on it…& make sure it’s seated all the way down the splines

Here's a good write up here that might help. It's for a D2 but same transfer box -

I reckon you are into a new tfer box there.
They are normally tough units, so a 2nd hand one should be a good bet.