Hi there
Well i know what it is your either tightening them too much or the internal gears are worn out . The larger diff case dosent match the one in the manual because its a larger one off a lst2 .they probably upgraded as the stock one does tend to break . The lat one has the metal inserts and uses the same gears as the other one . So try a different set of internal gears.
Make sure on the internal gear shafts that the ridges in them are sitting directly on eachother