OK, a few months ago I ordered two plastic ball diffs from Hobbyking, and they shipped me two gear diffs with metal ring gears and metal outdrives. Go figure, I sh!t you not.

You need the shorty metal input gear and shorty metal input shaft like mentioned.

I have ripped the stock plastic ring gear along the holes for the balls. All the way around. With a 13.5 on blinky and sweep slicks, sticky track. Never ever damaged any plastic teeth.

Never had a driveline prob with 21.5 nor 17.5, other than me over tightening the ball diff. But the plastic outdrives do wear where the pins make contact.