Wow sounds like jack is a spoiled brat I have rhodesians but our last rescue was a lab, who eventually became a wonderful nanny dog teaching soft mouth etc to our RR pups. two things come to mind to start, have you taught Jack a dominance down? I don't do this with every dog , mostly spoiled ones...

3 of my labs have done agaility for fun and confidence building at My Dogs Place in Mystic, CT with Bea ( http://www.mydogsplace.com ). They love it! My labs stay fit and trim. You can check them out currently at http://www.dogster.com/family/221417 , while I build my community.dog.com page. Lab owners...