np231 problem

I have an 88 xj with an AW4 transmission mated to the np231AMC. When I go from drive to reverse I get a loud CLANK in the transfer case. does not do it when you go from reverse to drive. I have taken the drive line out and checked the u-joints and they are solid. It does it in 4 wheel drive as well. Any suggestions as to what it might be, so when I tear this apart I know what I am looking for?

thanks for the input

Ed