Another problem/noise to add to the list

[Spencer]

Ok so cc goes out (will replace clock spring)
loud sqeek/rattle sounds like u joint, replaced rear with no luck
now as I turn showly I hear a popfrom the rear of my truck,

I'm hoping this is a u joint and not a differential.

I'm going to try to replace the front two u joints, however I hear mixed things such as there non replaceable.

If there replaceable where do I get these u joints from?