Generally if there is a "clunk" in the drivetrain it will show up when you put the car in gear..that is there are worn u-joints or excessive slop in the rear end..Sinc this only happens when you are driving I woudl look at shock mounts or shocks..maybe worn bushings in the suspension or something in that area..
My truck "clunks" but I have traced it to the shocks rattaling when I hit a bump..Remember these ole cars are not as smooth as we are used to in new cars..
And
aboard the board so to speak..
OMT