The person I bought it from had them replaced at the end of 2020. I did have my car sitting for a few months, which contributed to the rot, but the other 3 look fine (but what do I know). RWD 4.3L V6 LS (just added to my signature, thanks for the suggestion).RWD/AWD?
Depends on the overall condition/age/wear of the other tires. If they aren't in good shape or are super old, I would replace them all. With an AWD it's important that the tire circumference matches closely all around.
RWD and the rest are in great shape with plenty of tread then you might be able to replace 1 tire if they are close enough. Or maybe replace 2 of them, keeping the worst one for a full sized spare.
Not sure what would be the newer version of that tire, could ask the MFG.
If you buy them online you can have them installed at virtually any tire/repair shop. Some might charge you more for bringing in your own tires. I usually use Walmart(because they are nation wide and I usually get the lifetime rotation/balancing) or a small local shop to get tires installed.
Also, while you did put the year in your post, it helps if you put details like your year/model/engine/awd/conversion/etc in your signature line or your profile so we know what we are working with in the future. https://www.astrosafari.com/account/signature