ok your gear drive or stick shift trucks didn't seem to have a clunk , but the automatics do/did , here A reason why , the ring and pinion are in contact , passing thru neutral to change direction with a "gear " drive takes enough time , the ring/pinion have time for the backlash ( clearance between gears ) to center , when next direction is applied it takes up 1/2 the lash distance and the r+p are again in contact . with an automatic , even stopped in forward there is pressure on the r+p ,( often the same with "park" ) the full amount of back lash is behind that contact of the r+p , the auto trans changes from forward contact to reverse contact allowing the full backlash clearance to be taken up rapidly , making a clunk . your operation of a clutch , the gradual engagement takes up that back lash slower , and you usually are slightly stepping on the gas . that back lash if taken up rapidly , like "DROPPING" the clutch is what helps create "shock load" and "blows" the r+p when you do that ...