Not only do the gears have to be close, they have to be sort of seperate too. It is called meshing, or mesh. There has to be a minor space between the gears or else they will bind, if too far away, they won't transfer power properly and the power will go to the tips of the gear teeth and snap them off. You can try a few things: Your diff may be too tight, your slipper may be too tight (when you land the wheels spin at a different speed than the clutchbell is spinning, and you can shred gears if your slipper doesn't slip!) or it might be your meshing.

When do they break? What surface are you driving on? Does it break immediately? Do you know how to check your slipper and diff?
If you replaced a diff gear before, you may have it too tight (which would be why you had to replace the last one most likely) So I am goingto say that is your problem, but give us a little more info.