What do you all thinks wrong? Nd how would I fix it?

I have a '81 Chevy K20 with a 6" lift. As I'm driving down the truck will automatically drop into neutral and I hear this tiny clicking sound. To fix it, I pull over, put in N, turn it off, put transfer case in 4L then back to 2H. This happens usually when I hit a bump or slow down to fast, sometimes automatically.