Hi There, sorry to hear you have trouble, Ime sure there are far better qualified in this forum to answer this one , but I will list what I think would cause a knock from rear of truck when drive taken up. Ime going to assume its not something silly like the exhaust hitting the body or the suspension on the floor...
1, Check the condition of rear prop shaft and UV joints, check the nuts are tight and look for any signs of damage ect..
2, Check the rear trailing arm bushes, if they are badly worn this will allow the rear axle to float/move when when drive taken up.
3, The rear axle itself may have excessive backlash, this would probably require someone with mechanical know how to check for you.
4 Lastly the noise may be comming from somwhere else, it could be your gearbox layshaft, is the box in good nick , they all are a bit noisey but have you checked yours oot?
Well thats enough for now, ime sure the rest of the lads will pile in soon, cheers