The lights are controlled by an electronic gizmo that compares the primary VSS signal with the 4wd VSS signal, the latter usually being on the back end of the front output shaft. If you aren't moving, the light will stay on forever, because there's no diff in the signal. If there no diff in the signal, the front lights are lit, diff in signal, the lights go out, and in some late models, if there's any front drive signal, the lights stay lit.

Not responsible for advice not taken...