Thank you much for the tips. The 272R gearbox is available for around $120-ish online, a bit pricy but as you eluded it might actually be the less expensive way to go if things keep breaking. We've got 4 of these things to upgrade so it adds up quick. Do you know if it's a direct drop-in, no mods required?
A new metal diff is on the way, I'm stabbing in the dark that the plastic one flexed a little and contributed to the gears failing, but I really don't think that was it. Really I wanted to try thick oil in it to see if I can make it behave like a limited-slip diff.
But looking at the failure carefully, the gears on the output shafts are very thin and only engage about 1/2 the width of the teeth on the planet gears, and that happens to be the 1/2 of the teeth that sheared off (image below). This makes me think that output shafts with wider gears for more engagement might actually be the better thing to try, if I can find such output shafts. Parts on the way, I'll try loosening the slipper and turn down the motor a tad and see where that takes me, but maybe the 272R could be worth a try if things keep breaking.
