On my 87 5kTQ, my display has TWO chassis. The one on the left has a green
light for the centre diff, and the one on the right has a green light for the
rear diff. When I had it apart, the bulbs appeared to be a little different,
but only in shape (they were rounded at the top, where the running lights were
squared off.) The green, I'm pretty sure, is a product of colored plastic on
the display itself, not the bulb. Definately worth checking into. Also, to
lock the diffs on mine you TURN the knob, not pull it out. Of course, YMMV.
87 5000CS TQ