Well speaking from experience since we have the same trucks, and I build these trannys quite regular, I do know that the A wire is hooked to the 4-3 switch, B is hooked to the 4th cl. switch and D is hooked to 3rd cl. switch. If you still have all original wiring, and who ever built it, put in all the switches yours has, you will have lockup in 2nd 3rd and 4th. But unless you pull the pan you will not know for sure. Most people think you only need lockup in 4th and to that I say
You need at least 3rd and 4th, and if you work yours as hard as I do mine on occasion, yes you need even 2nd. And as I said if no one has butchered your original wiring you are already wired through the brake switch and when you put the transfer case in either lock position it should unlock it too. Good luck with your new one, if you have any questions by all means ask and i'll do my best to help you out. And if they built yours as good as the one I just put in my old 74 C 20 you'll be able to do this just like like me.
