A morph doesn't affect the JPs, so due to the large change in size your bound to get problems.

What might work better is to use the dials to scale David down to approximately Luke's size (for all body parts), save and apply this OBJ as a morph, and then reverse the scaling with dials (there'll be some calcuation involved).

Other alternatives would be to either use D|S's "Figure Mixer", or adjust the rigging on your current morphed figure.