You don't need a gear diff, you just need a real thrust bearing to replace the junk Associated sold you.
Correct Size Thrust Bearing
And don't put too much of the white silicone grease, or any black grease at all, in the diff balls. This is usually the case when I see people who are racing pan cars and say that their diff is so tight you couldn't pull a pencil out of it with a bulldozer but still slips, lithium/moly/carbon black grease is not your friend in the diff.