factors that play into critical speed are , diameter, length, material, trans gear ratio, rearend ratio.
Aluminum has about a 20% higher critical speed than steel, so they say. Critical speed is the rpm at which a driveshaft can spin before it begins to wobble in the middle and then break, which happened to this one.
It had nothing to do with power, it had to do with the shaft spinning faster due to the 6speed auto's gear reduction (engine spins slower, so something has to spin faster).

Aluminum without a doubt is weaker than steel, however, this truck has 4.56 gears, a 6speed auto and the engine turns high rpm, and with aluminum having a higher cs, that's one reason why an alum was chosen. It needs a shaft that will take the rpms and not break.
If there was a 5" diameter shaft we'd get that one!