Sorry for the long post but a little more detail on rebuilding an ball diff
The link above is the best way to build silky smooth diffs I use ceramics balls instead of steel as the are less likely to flat spot and bed in quicker due to being harder than steel or carbide.
Note: This process take a bit of time but once done your diff can last a long time before needing a rebuild! And you just need to sand the groove out of the ring rather than buy a new set.
I would sand both sides of the diff rings with 400 gritwet and dry so that there are no low spots on the rings by sanding them in a circular motion on a flat surface due to the pressing process warps them slightly.This gives an even thrust spread across the balls. When tightening you will find that, you don't need to tighten the diff as would previouly to compensate for the uneven thrust load. Then i would finish with 1000 1200 grit wet and dry this give the diff rings a keyed surface so that the ball will roll rather than side over the diff rings. Unkeyed ring require more force to stop them slipping and cause diff to over heat and flat spot the balls if not bedded in.
Note: Clean the rings and hands after sanding!!!
To grease the balls; Note the grease is to act as an coolant rather than a lubricant. It also acts as friction inducer the stickyness of the grease allow the ball to roll against the ring rather than slip. You should lubricate the ball by putting a little grease on to finger and thumb(clean finger and thumb) and roll each ball betteen your finger and thumb to get the right amount of grease on before insersting in to the diff gear no more is need after that.
Thrust bearing the smaller one is under a high load of the diff screw it job is to stop the screw tightening or backing out of the diff it need a as much black grease or what ever the manufactures recomend to keep it cool and lubricate it the sheer force on the thrust bearing will cause it to slip ratehr than roll that why you pack it with grease.
I would bed it in on the bench with the diff set to slip slightly this is done to being to key a groove in the ring. Do this for 20sec on full throttle left wheel and right. To set the slip; tighten the diff down so it won't slip then back it off 1/4 turn.
Doing it with the diff fully tightend causes the balls to flat spot and slip.
Then i would tighten the diff (s) to so they won't slip at the with your thumb pushing the diff while hold the tyres down do the same for the front diff run it and tighten the front down to reduce oversteer.
Most people run the front diff tighter than the rear to reduce oversteer and stop it slipping to much on exit of the turn.
The end result is you should be able to spin a wheel and it should rotate the both in opposite directions for a revoultion or two.