Breaking cranks may be a sign of another problem, like transmission issues or a harmonic balancer issue.
Harmonic balancer issues are not entirely unknown (de-lamination of the outer portion from the inner, and sheared keyway on the crank - in both revisions, early and late) and there are issues related to the torque converter clutch lockup (it never locks up fully - it's always partial)
The torque converter issues result in a couple of manifestations or phenomena - the first being "RSN" or
"rumble strip noise" (so named since it is similar to
driving over rumble strips), which is apparently caused (or worsened) by degraded transmission fluid - since changing fluid will often eliminate it. It is a slight shuddering or chattering of the torque converter clutch. It can occur on both acceleration or deceleration.
The other phenomena is somewhat similar, but it is
far more violent - an extreme shuddering .... which likely transmits throughout the driveline and engine. It tends to happen when the transmission is, or remains in, a higher gear than what is appropriate for the conditions (load, grade, etc) at the moment. When this occurs, the engine is generally turning fairly low rpms (under 1500 to 2000) and is lugging.
The first condition (RSN) is generally thought to be fairly harmless ..... the second condition - the
violent shuddering - is, in the opinion of some (myself included) considered to be
potentially harmful.
The second condition can be alleviated by immediately "getting off the pedal" and/or downshifting to a more appropriate gear for the conditions.
Avoid the second condition always, immediately as it occurs, and don't flog the vehicle.