Not saying that it isn't possible, but for the same company to be having the same issue with multiple trucks, could indicate that maybe something isn't right with the use of the vehicle. ( possibly not big enough for the load maybe )
It is possible the repair facility is mis-diagnosing the failures as well. Most GM dealerships are not authorized Allison shops (nor am I).
I have yet to see an Allison 1000 fail to the point it needs to be replaced, there were a few in '04 that had issues with the valve bodies, and there is a TSB about fluid drainback from the converter when sitting over night on some of the 03-04 models. (would result in not moving right after startup) requiring pump replacment, I've seen a few failures of the PRNDL switch (aka NSBU) on the side of the trans, which tells the TCM what position the shift selector is in. Some of these could result in not moving or funky, irratic shifting.
I have seen a few of the 4500/5500 trucks blow out a trans cooler line from the trans, emptying the trans and stop moving. But even these, repair the line and fittings, fill them back up with DEX3 and they move on down the road with no issues. ( Same engine/trans as in the 2500/3500 truck, mount positions are different, some programing differances )
I'd be curious at what mileage and what sort of loads they were pulling. Not to mention how the drivers were hammering on them.
Kevin