Changing differential gear ration by using different pinion.

[Patrick]

A 10t pinion of the same pitch as a 13t will have a smaller diameter, and might not be big enough to reach the ring gear enough to mesh properly inside the diff case. I haven't looked closely before, but I would say ring gears are off set differently to suit 10t or 13t pinions.
If you have a lot of room if the diff housing you may be able to shim the gears close enough, worth trying anyway.

Have you counted the teeth on the st rr pinion? I'm pretty sure the truggy has a 10t pinion anyway, or did they get swapped for buggy diffs?