At the top of the 750A page - under schematics - is a change relay schematic. Although this is for a 76, it is the same (as far as the gear lights) as for a 77.
If the neutral light isn't going out, and your L and D lights aren't coming on, then my first check would be the Changing Switch. This switch is located under the left sprocket cover - but there should be two wires coming out that area that not part of the plug (that also comes out of that area). The two wires have individual bullet connectors on them. The wire colors are Brown/Red and Blue/Yellow. When measured to ground, the Blue/Yellow should be shorted to ground while in Low (open otherwise); and Brown/Red should be shorted to ground while in High (open otherwise).
