Transfer Noise

I've had this 1999 Disco II (clean, 60k) miles for about 4 months and it did have a 'wine', kinda like a worn diff, from the time I bought it. Now I'm getting a pretty loud tick or clicking when driving (forward or reverse) that quiests somewhat when I let off the gas. When I put the transfer in low range this noise disappears but returns as soon as the tranfer is shifted back into high? Any ideas about this? I found a used transfer on the net but they state if you need it with diff lock it's an extra $50. How can I tell if I have a diff lock and if I need it? Thanks so much for your help.