rear end/suspension cracking sound need help

So i swapped my stock rear end with one my bro gave me its a full posi 3.73 gears i swapped everything when i say everything i mean upper and lower arms and 2" lowering eibach springs and isolaters 2 days later i have a cracking sound only at slow speeds of 1 to 10 mph and mostly as im turning into a street or backing up into my driveway i havnt gotten a chance to open up the diff, i turned the wheels by hand and i dont hear no noise also lifted the cars on stands andput it in gears drive and reverse and still no noise just as i drive i even changed both u joints i need some help

Ok as soon as im able too ill do so, can i buy a bottle of the modifier at auto zone its my first time dealing with a posi, and i hope that might be the issue because i do recall my bro saying he had flushed the diff and put in some limited slip fluid in it