First place to start is the transfer box input gear, but as yours is a later car it might already have the cross drilled type fitted.
This is if the clonk sounds like its coming up from under the centre seat?
Lynall
I'd go for either A Frame or more likely rear prop UJ
Lynall mines coming from there? am trying everything to pin point it!! started after i came back from laning last saturday!! clonks when i change gear 2nd and 3rd??? any advise?? 2000 model td5
drive shaft uj's when you step on it they get tight when you shift the power is off and got wear in them. grease everytime you return from 4 wheelin and they last for ever. no grease and go to **** in a hurry.