well as trizo said get a new center/part thats screwed as i for sure wouldn't be welding it for a street car...track car maybe cause its very dangerous welding it for street use
but yeh get under there and check out the tail shaft center bearing..the rubber surround may be stuffed, also check the uni's
and lol (how could i forget) theres two upper arms that hold the diff to the car that have 2 rubbers either end...they look like a dog bone shape. these rubbers flog out pretty commonly so i wouldn't be s surprised if they are stuffed or close to it causeing the clunk.
well both wheels would be going at the same time with teh same amout of force so yeh add two and two togeather and i can see there will be problems.....your better off (if it is teh diff) getting a complete diff from t6eh wreckers