When turing slowly left or right i hear a slight squeeking / rubbing coming from the rear end its not very loud but noticable at low speeds seem to come and go whenever it pleases sometimes it does it and sometimes it wont. Does this sound like the posi isnt unlocking or something.
I was thinking changing the fluid and putting some additive in there or do you tihnk its a more serious problem

 

If you do a search to confirm this, you will find that this is a fairly common problem. Most find a solution in changing the differential fluid and adding a bottle of GM addititive when you do. Seemed to have fixed mine when I was having the same problem. Some will also recommend not putting a synthetic fluid in when you change it out.