DIFF BUILDING TIPS
The Ceramic or Carbide Diff Balls would be near impossible to find in the US in bulk as they are 3.5mm size. We would recommend getting either of these as an upgrade. Like Ray mentioned though the cost factor to include them in a kit is tough. Also, you would not need these in bulk really as the Ceramic or Carbide balls will last months and months.
Out of the box we recommend pulling out the snap ring and packing the thrust with black grease. Then put it back together, make sure the ring is in the groove (Ray
Reassemble the diff and use 6 of the beveled washers on the screw. Snug it just enough so it doesn't fall apart. Put the bulkheads back on and assemble with tires. Adjust the diff with tires on and snug up little by little. DO NOT OVERTIGHTEN. Check the diff tension as usual, but do not use a Gorilla Grip and force it to slip. I repeat, DO NO OVERTIGHTEN.
Diff should be pretty much be locked and still feel smooth as butter.