If ujoints are bad they won't hold the joint in place tightly. So when torque is applied it'll let it slip as the bearings are no longer tightly packed. That could be the clunk. Usually you'll get some vibrations through driveshaft if ujoints go bad.
Mine was caused by the spider gears being worn real bad, The mechanic that went to install my Detroit Truetrac showed me hou bad they were. The Truetrac eliminated these parts so all was fixed, The problem was worst when actually leaving a stop.