A friend of mine has a similar problem, and used a hardened pin (Team Associated part) in the transmission to keep that from happening.
Apologies that I do not have better information, but I would recommend taking the sheared pin and transmission to your hobby shop and approximate the size with the Team Associated equivalent part.. it is a hardened pin (not a roll pin)
Also happens much less frequently after loosening the slipper clutch to allow a little bit of slip with that much power. The Differential is the next weakest link, and we have shredded Diff gears as well. The Traxxas rustler transmission (used in slash) is one of the oldest and most bashed transmissions EVER and the beef of the gears is not really an issue.. you broke a cheap little part that kept you from shredding a diff.
Good luck!