Problems with OEM driveshafts?

I had my '94 C1500 in the shop for it's yearly state inspection, and the mechanic told me to replace the driveshaft as soon as possible as it was cracked, and "spider webbed" all along the entire unit. He said he has seen a lot of these as the aluminum and magnesium used does not hold up well. He recommended replacing the shaft with a steel unit, or get another aluminum one. I called around to get pricing, and found out that in my area, the aluminum units are hard to come by used as when the truck is in the salvage yard, they tend to be bad already. I ended up taking my driveshaft to the local shop that does work on these, and had it built into a steel unit using the same ends from my old shaft. This ran about $225. I wonder what caused the shaft to go bad in the first place as I really don't tow with the truck and I haul mulch and compost during the warmer months. I would think the driveshaft could handle work like that. I have heard one thing that if the shaft should develop a crack and water should get in there and freeze, then that could worsen the effect. I understand GM's use of the aluminum and magnesium to lower vehicle weight for better gas mileage. I wasn't even aware I had an aluminum driveshaft as all the other vehicles I have owned and worked on had steel ones. Has anyone else had problems with their aluminum driveshafts, or know what causes these units to go bad?