The diffs should hold up fine if they are shimmed correctly or even shimmed a little tight. I suspect that the problem is the gears are worn down or damaged from the previous breakages.
I had the same problem and the way that I fixed it was to rebuild the diffs with new gears and then I shimmed it a little tight. No problem since.

As far as dyno's go there is no set standard for dynoing a motor in the RC industry. Two identical motors will give different numbers from different manufacturers dyno's. This will always be a problem until a standard is settled upon.
Poor judgement is correct when he mention the torque difference. The .12 might have more horse power but the .18 will have a whole lot more torque.


rolland