Shim.

Shim at RC Shopper: 31 products listed, shipped from the UK. Prices run from £2.75 to £6.99. Stock comes from MST and Avid. Fitment varies, so read the listing for the exact chassis and revision. What is in stock is on the shelf, not on order from a distributor. Shims are how you take slop out of a drivetrain without buying new parts. Cheap, and usually the actual fix.