The way I go about trying to troubleshoot stuff like this is to begin by eliminating as much stuff as possible.
Does it only do it when you are rolling? If it will do it sitting still in neutral, you've eliminated the tranny, CV joint, wheel bearings, etc. . . .
If it's engine RPM related and not vehicle speed related, you probably eliminate those things too. Get a helper to try to rock the engine back and forth and check all the mounts. If you're ignition is breaking down under a load it might cause it (bad plug or plug wire, points floating, etc.).
If it only does it going down the road, get on a long flat straightaway and get it doing it . . . what happens when you mash the clutch?
Like Herb said . . . that's a tough one without knowing more about it or actually hearing/feeling it.
