At the front of the driveshaft, there is a rubber coupler, called the "flex disc", instead of a U joint. They are in a way considered a "wear item". I wouldn't be surprised if this is the cause of the clunk. The rear of the shaft typically has a hard rubber coupler, and they seldon wear out, but the front one has some cushioned rubber areas around all the bolts which hold the coupler to the transmission output yoke and the driveshaft yoke. It helps make the drivetrain power transmission softer, which helps make a more comfortable ride and absorbs shocks in the entire driveline.
When the cushioning wears out, it makes the driveshft kind of sloppy and can cause the clunk when shifting from drive to reverse/reverse to drive (change in shaft direction).