'93 Ford F-150 4x4 dash light

The light on the dash that's supposed to light up when the transfer box is shifted into 4x4 no longer lights. Opened the dash and replaced the bulb but it still does not light. Tried wiggling the electrical plug to the transfer case and the light started working, but after running over a few bumps in the road the light no longer lights. Does anyone know what the part is that the plug connects to on the transfer case so I can try replacing that part?