Newbies, take a ball diff apart. Remove the D-rings (big silver washers that the balls in the gear touch and make a little groove) clean them up, and get a piece of cardheader (the small cardboard piece that usually comes stapled on the top of a R/C part with the item name/description on it) now glue this cardheader piece to the D-rings (both), let dry. After dry trim the card to the same shape and size as the ring (should be easy with it glued on) next remove all diff balls out of the gear only! You do not need to remove the thrust washers or bearings. Reassemble with the cardheader facing the gear (both D-rings), and reassemble and tighten the diff bolt till it is almost completely tight.
What this makes is essentially a slipper clutch, but you dont want it to slip until it needs to. I like this idea better than the full blown spool since this can break loose if you hit the pipes under power, saving your drive shafts and outdrives, and gears if you have a shaft car.