When you consider all the wrenchin involved to change just the tie rods...its not much more work to swap the entire rack. I've replaced several rack units and you basically open the box and bolt it on. All of the replacement racks I've seen came with 16mm rods, and they are pre-adjusted and ready to run. Only one time did I have to make an adjustment...(one turn on the tie-rod), and I think that was due to worn A-arm bushings on that truck. IMO...I would go for the total replacement, not only because its an easy swap...but you can use your old rack & tie-rods for trail spares. Racks are only $275 or so, and that includes 16mm innner & outers. Not sure what (2) inners and (2) outers cost separately...but it might work out close to the same price, by the time you have to jack with swapping and adjusting all four individual TREs.
The whole assembly is considered to be a "component", and it comes that way for two reasons...LIABILITY and SHOP TIME. Its just much quicker to change the whole enchilada than it is to trouble-shoot, diagnose, and swap individual pieces. Not only that but, when the tech is done with the repair, he can rest assured that the ENTIRE steering system is fixed and that vehicle will not come back into his shop. This is why its sometimes hard to find individual tie-rods at the dealer, and also why I've never heard of a dealer swapping a single tie-rod. I'm pretty critical of GM...but I gotta admit, they got "component-izing" down to a science. I'm sure they have done studies on such repairs and they've determined that this is the cheaper fix. Otherwise it wouldn't come as a complete assembly.
Best of all...by swapping the entire rack, you can also do the bushing fix (of your choice) at the same time!