I would seriously consider one of the rear axels. Most other things that could be the causitive agent are either constantly good or constantly bad. Those cv joints are kind of notorious for intermittencies until they become constant problems eventually. You might get under the car and push and pull the shafts laterally to see if one has a lot more movement than the other. I assume the axel boots had no leakage when at the garage or they would have spotted that. Your getting rid of the noise temporarily by backing up was interesting. Also a good way to drop a ball out of a well worn cv joint sometimes. I cannot also not recall any other part in the area you are looking into going clunk, clunk, clunk, unless almost totally broken at low speed. Changing one axel from a junkyard source is not a big expensive deal either yourself if you can get some indication of the affected side. Since you have just moved why not try to find a site member in your area? At the least he may be able to steer you away from the local bandits. If not generally give you some direct assistance. Again I never bet on anything but if I did the odds on a bad axel cv joint in your case are pretty fair.. Once you go through a few failing cv joints on a lot of cars you will understand where I am coming from. Cv joints can be somewhat devious at times in my opinion.