I have a ford 1220 that has served me well. Lately the front end is clicking and grinding when 4wd is engaged. If I hook up to something and try to tow it it just clicks wont move it.

Anyway I assume I am in for some front end work. Can anyone tell me where to start? What wears out?
I would prefer to have parts ready to install before I tear it down. I cannot afford to have the machine laid up. I use her for residential plowing and mowing.

Any help appreciated.