Clunking sound when switching in to four wheel drive low

When I shift into four wheel low I hear a small clunking sound. I had it checked at a Ford dealer and they tried a used transfer case but it had the same problem. They told me they had several other rangers with the same problem and could not find an answer. I told them I shifted into four wheel low by stopping and putting it in neutral or park and was told they did the same thing with the same problem. The problem continues.