Sounds as if the wrong switch has been used. It's a very simple thing to do your self. If you take the panel under the middle seat/ cubby box off you will see two switches with cables comming off them. The one directly below you comming off the back of the main box is the reversing light switch. The one on the transfer box is the diff lock one. They just screw in, and you have o adjust them so that the lights come on and off as you move the selectors. It's basicly just a push button switch, and when you move the selector it either pushes the switch in or not.
It sounds however that he may have used a reefing light switch for the diff lock, as I think one is normal open, while the other in normally closed(but I may be wrong here)
Wittier take it back to him and get him to do the job properly or just get the correct bit from eBay Nd fit it your self it's a ten min job.
Just to clarify however that the light should not change between L/H only between lock and unlock.
You may have to drive a bit to make it come on as it needs a bit if movement to make it lock.
The other option would be to fit some electonics into the circuit ( either a transistor circuit or a relay which will act as a Not gate and basily switch around the logic of the circuit I.e make on become of and off become on)