If there are only two holes in your spur gear I'm thinking you actually have a spur for a TC and those are mounting holes. Two holes is not going to work effectively for several reasons.

1) the diff rings are "balanced" on a line and are likely able to rock/tip back and forth. I would not be surprised if they actually "escaped" their keying (the 'D') with this. I would think that 3 balls (creating a triangle thus not allowing the rings to tip) would be the minimum that would even work reliably.

2) as mentioned before, the way a diff works is by having enough friction for the wheels to turn forward under power. The more diff balls (contact points) you have the less you have to tighten the diff to get the same amount of friction. 12 balls require roughly half the pressure as 6 balls and allows a noticeably smoother/freeer diff action. I can't imagine what 2 balls (in the diff, sickos ) would feel like.

Do yourself a favor and spring $5-6 for a proper spur gear and another couple bucks on enough diff balls to fill it. You will feel the difference as soon as it's built even if you don't sand the diff rings, etc.