jack car up ! grab the shaft, grab the uni joint, if there is any movement there then you will have some degree of clunk cos if you can do it by hand, then under huge forces from the box, it will be a lot more.. on a side note, I used to have a clunk in my xg ute, turned out to a be a flogged out diff ! just worn out ! but I don't suspect this with yours at all.. could be the exhaust thumping something underneath when you boot it, or driveline just hitting something too close that needs adjusted further away.. our brand new FG will clunk a bit of you misuse/poorly time gearchanges etc.. my EF wagon is smoother !