I tend to agree with 130cc - I have just replaced the rear diff in my Freelander and all the mountings were shot which caused knocking on changing down but never a vibration.
Check - wheel balance
Tyres - can be out of round
The Freelander's tail shaft spins at a relatively high speed and has a heavy viscous coupling on it that requires a huge heavy damper on it to cancel out the vibrations - the workshop manual warns about ensuring the tailshaft remains in balance when being worked on by putting alignment marks on it - so the balance of the shaft is clearly very critical - however I just but in new UJs and a different VC in mine and I had no problems. So if the tyres are OK and all the wheels are in balance - I would be looking at the balance of the tailshaft - are the UJs OK - is the tail shaft CV ok - are the VC bearings OK.
Failing that - wheels bearings but there should be other symptoms or CVs but that is most unlikely.
Other alternatives - engine mountings allowing engine vibration - most unlikely - was a problem in my new disco 10 years ago - vibration at 2500 rpm - fixed with new engine mounts.
Obvios thing first - wheels in balance, tyres out of round (flatspotted), tailshaft balance, wheels bearings/CVs but there would be other symptoms.
Hope this helps
Garry
Canberra