Could be transfer case, could also be u-joints in the drive shaft. If you haven't replaced those in a while it's good, cheap preventative maintenance and could save you more damage in the future. There are several threads in the archives about how to do this job and what parts to get. I think I even have a Word doc somewhere detailing the procedure with photographs. If you can verify this is the problem then send me a pm with your email address and I'll send the write up. That goes for anyone on here, by the way.