Blowing 4wd fuse 420d

hey guys I have a cat 420d 02 model anyhow my 4wd is not working. It started out popping a fuse every once in a while now whenever you flip the switch for 4wd it blows the fuse. What do I need to do as far as trouble shooting to track down the problem. I have searched the forum at no success any help would be greatly appreciated

Blowing 4wd fuse 420d