you can try several things:
1. sand the input gear a bit as well, make sure you do it evenly
2. remove the center shaft first so it's just the diffs in the bulkhead, and then spin them...they should spin very free. If they don't, then the bulkheads are not aligned properly. And there is a simple solution for that:
2A: first loosen all the screw underneath the bulkheads (front and rear) by 2 turns so the bulkhead is free. At this point, the diff should spin very free.
2B: then you put the center shaft in place, with the plastic center shaft holders in place on the bulkheads. Make sure the shaft holders are seated properly on the bulkhead. Now, hold the center shaft holders in place with one hand and began slowly screwing in the bottom bulkhead screws. Make sure you do each screw evenly and a little bit at a time (don't screw down one screw all the way, then the others...do a "star" pattern) Your bulkheads now should be properly aligned with the rest of the drivetrain.
I had a customer that had the same problem, and we took care of it with the method above
3. Another thing you might want to try is different shimming. And this varies slightly with each car. For my car, I have one shim on each side of the diff, while in the front I have both on one side, shimming the diff gear away from the input gear.
Try these methods above...I assure you, when you build the barracuda R2 correctly, the drive train is amazing!
Right now, my drivetrain is so free to a point where if I rotate both front wheel forward (which's got a one way, so the rotating it forward does not rotate the rear wheel) as it spins forward, the friction created in the one-way bearings is enough to bring the rest of the transmission and the rear tire to start spinning.
I will get a lot of building tips and videos of my drivetrain on the www.alexracingusa.com
website in the next couple of weeks
Thanks for reading, I apologize for another long post