2000 International Truck Replacing Pistons

Without knowing what caused the piston failure, there's no way to know if the others were affected the same way. If one piston cracked, you could have an ignition timing problem that must be addressed first to prevent a repeat failure. If it is scuffed, that's from an oiling problem that would also affect all the pistons.

The reason for replacing all the pistons as a set is because they're matched for weight. That becomes more important at higher speeds. For a truck engine, you typically won't hit real high engine speeds often, and when you do, you aren't likely to notice a vibration as long as you stick with a stock, factory piston that is of the same design as the original pistons.

The reason for the problem was the oil cooling tube. When we dropped the oil pan we noticed that it was in the oil pan along with some fragments from the piston.

I would be inclined to replace just the one piston as long as you can find one with the same clearance. I'd still question what caused the failure. If the dealer indicates there has been a common problem, then consider replacing them all as long as everything is apart.