I had a similiar clunk and it was due to me putting too much grease in the slip joint zerk on the rear driveshaft. To fix it, I removed the slip joint and cleaned out all of the excessive grease. I then just brushed on some grease onto the teeth of the joint. I also left the zerk fitting out until I found a "breather" fitting to replace the zerk fitting. Basically too much grease doesn't allow the joint to move back and forth easily which can cause binding in the drive train. This has been a problem with other people as well. Mine hasn't clunked once since I made the changes.

May not be your "clunk" but it's worth taking a look at.