My rustler running 4s lipo on an 8L with maxx size slicks in the back has never broken the diff internals.
Im guessing you guys that keep breaking them are doing it when one wheel lifts and the diff unloads then comes back down? You could try changing your suspension setup to avoid this. Softer rear springs, more weight over the rear, or heavier rear wheels/ tires would all help. Heavy grease in the diff should also help reduce the unloading. Also the slight flex of plastic driveshafts might help the diff survive. I doubt the stock rustler shafts would survive anything that the diff internals wont, but old style maxx driveshafts are a direct fit if you swap to 6x11 hub bearings. The stock 12mm hex can then be drilled out to 6mm, or you can swap to maxx rear hubs and wheels.
Or you could always adjust the slipper so it does its job.