My IHC D200 has an indicator lamp as do many other vehicles of the same time period. Later GM vehicles as well. There were also aftermarket indicator kits.
The factory installed NP205 switches are very hard to find and require a special shift rail that has a shallow center notch. If your case has tapped swith provisions (#32) Rail detail see here at quad
http://www.quad4x4.com/images/qu30069notchdetail.jpg
But overall the concept is simple. A N.O. switch grounding a dash lamp with an ignition hot (B+). Should be easy enought to find a waterproof switch and make an external bracket.
Or a less elegant but workable solution would be a floor mount gate to prevent shifter movement that you could flip to engage 4X4. If You REALLY need me to I can try to take and post the setup on my remote NP205.