When i rebuild my diff i start with cleaning everything
Then i sand the diff rings till i cannot see the lines on the diff rings , then i add a small smear if Assosiated diff fat to both sides of the diff rings and into the holes of the diff balls NOT A LOT this is good stuff but if you add too much it will spray out everywhere attracting dirt and dust.
Add asso fat to the thrust bearing , and rebuild this to the diff half
Then i add Hudy bearing oil to the bearings (this is THE BEST oil in the world !!)
Then add the diff balls into the pulley and screw the 2 pieces together
I leave the diff as loose as i can get away with then after fitting it to the car i hold one wheel and give a GENTLE !! press on the throttle at LOW speed then the same on other wheel, this beds the diff in slightly , then i hold both rear wheels and try to turn the spur gear and adjust if necessary, you need to think about how much torque your motor has in comparison to your finger .
I.E. if you run 21 class then your motor wont slip the diff as easily as a mod motor, so you can run it looser , i tighten the dif so i can move the spur slightly with my finger , thus a small amount of slippage , but during my first battery i try to listen to the car out of the corners to hear for the motor reving and diff slipping if this is happening i will stop and adjust it a small amount to stop it.
My diff is so loose usually i can spin one wheel's with little or no resistance
It is a difficult thing to get good but if you can master it then you have a loverly smooth diff and unwanted no slipping.
Hope you understand my jibberish