had the same symptoms on my 99 Forester, clunking on a very slow tight turn..noticed it first in a parking garage doing slow tight turns around corners. I have a MT so not sure if this will work with yours but it was suggested that I try doing figure 8's in reverse maybey 10-15 of them. The clunking disappeared and I then had the diff oil changed and the clunking has not reared its ugly head since.

check the driveshaft U-joints. they will be starting to get sloppy if they are bad