Yeah mate its pretty common with the d40 's,
mine was replaced at 50000kms, and mine is fully loaded everyday, so you've killed yours pretty quick.
You really need to get used to the power delivery, or lack of on take off, and try not launch it to compensate for the lack of low down power.
Not sure what you drove before, but I came from a heavily worked ss ute which had ridculous power (260 kw at the wheels) and found myself slipping the clutch on the D40 a little to get it moving quickly.
What you've got to remember is that they weigh over 2 tonne and are only a little engine, with a turbo that doesn't produce any real power until 2 grand, so my advise to you is to try and change your driving style a little and be a bit easier on the clutch on take off, but if you think that its still slipping now then I'd say bite the bullet and put a heavy duty aftermarket clutch in it and be done, I will defineately be doing that with mine next time.
I think that it has some to do with the fact they they have a rubber mounted dual mass flywheel, and the series 4 navara has gone to a solid flywheel, so you should have got them to put that one in when they did it, maybe they did, worth asking them!
