Mic,
Have you tried heating the roots? Bare root the trees, trim the roots with visible galls, remove thick roots, then soak the root mass in 120F/49C water for 10-30 minutes.
RKN can be found in two places. The adults embed themselves in the roots (producing galls) where they lay their eggs and live out the rest of their lives. The eggs are also inside the roots or partially exposed to the soil. Once they hatch, the juvenile spread out in the soil to find a root of their own. Bare-rooting the trees removes the juveniles trying to find a new home, and heating the roots kills the adults, as well as the eggs. If the roots are thin, a shorter soak time will do. If the roots are thicker, aim for a longer bath. Either way, maintain the temperature. A few degrees warmer probably will not hurt the tree, but I do not know what the upper limit is.
Contrary to what Gardengal has said, nematodes are carried by wind, water, clothes, shoes, tools, etc. So keeping infected soil/plants in quarantine is mandatory. I set up a secondary quarantine are for trees which had been treated until I verified the treatment worked. Your area might have a significant problem with RKN that it is impossible to eliminate them. I had a problem in Houston with my neighbor's gardening activities. When you do bare-root your trees, make sure you do this away from where your trees are growing and away from tools/workbenches that you might use to work on your trees. Discard the growing mix and containers (or heat treat the containers).