So you were thinking that the transmission shifting too hard caused the rear end to crack?
The tail housing geartrain endplay of these transmissions is around 0.015" or so. I can't imagine having this movement would damage the rear end, since there's a slip yoke that allows the driveshaft to "stretch." Was the yoke maybe binding? Was it perhaps a coinsidence or I'm missing something?
I'd think maybe a drum, shaft, sprag or bearing inside the transmission is what would fail if shifting is too harsh.
In any case, glad the problem was resolved.
The issue is not the length of the driveshaft, and the end-play of the transmission getting "too long".
The issue is the shock-loading at gear change: fairly high engine torque, slam-shift so that rotational inertia of the trans guts adds to the torque, plus the converter is probably forced into torque multiplication--all happening in a split-second. The shock of that instantaneous torque increase cracks the rear axle guts. Guys love that the shifts make the tires spin--sticky or heavily-loaded tires that
don't spin, don't reduce the torque going through the axle assembly.
Yes, all the rotating parts are at risk from the shock load--trans, driveshaft, axle. Not so much the engine, as there's no geared or hydraulic torque multiplication there. Turbocharged engines can be at risk, though--the turbo gets spooled-up, then engine RPM drops so there's a pressure surge at the shift point. The pressure surge massively increases cylinder pressure--and therefore load on the pistons, rods, and crank.