I don't know what the "the knock" is - but there's plenty of different "knocks" that people experience with Freelanders - and most (all?) do not relate to the IRD. They're more likely roll bar bushes, drop links etc etc. If you get a 'knock' from the IRD - its generally a very big one followed by immediate transmission failure
Personally, I think you'd be unwise to spend a grand replacing the IRD & VCU because you can hear a knock. I'd want a lot more proof first.
First thing I'd do is change the oil in the IRD. If it looks new - then it probably is and won't tell you much. If it just looks like used oil, then you're IRD is probably fine. If it looks grey/metalic then could very well be in need of a recon IRD.
Does the car feel like its binding ip on reverse with full lock? ie coast backward and turn to full lock - does it slow down more dramatically than expected? This might indicate you're VCU is going. Do a 1 wheel up test (see the cFAQ) - it will give you a very good idea if the VCU needs replacement. Some VCUs can go virtually indefinitely, others fail more quickly - presumably due to the life they've had. If yours is good and you replace it - you're throwing money away.
What is more immediately important than the VCU, are your tyres. Make sure they're all the same make/model and reasonably even wear. Definitely check the pressures in them - make sure they are all the same - its hugely important. I'd say more transmissions are stuffed by incorrect tyres than the VCU beginning to 'lock up'.
If you do need an IRD, I think the concensus is that its not worth going anywhere other than Bell Engineering. If you want a VCU, you have a choice of a new GKN unit (that are presumably the same where ever you buy them) or recon - be more careful with recon, once again I think Bell can help there.