2001 Chevy blazer used for field work - typically in 4WD, but not always. Do i need to buy ALL 4 Tires new because one is unrepairable?
NOPE.
Not as long as the new tire you put on is the same size and rating(speed and load) as the old ones.
There are exceptions though. With the 4WD being part time, you can avoid replacing all at the same time.
If the old tires are more than 1/3rd worn down, you might want to replace all of them. OR, find a good used one that closely matches the size and ratings of the existing tires…
It’s always preferable to match brands/treads/size/load and speed ratings whenever possible, AND to replace at LEAST both tires on the SAME axle.
Size is most important to avoid unnecessary wear on the differentials and transfer case.
No, but you do want tires on each axle to be nearly the same, like Roadrunner said. You can either buy one (if it’s matching model) and have it shaved, if needed, to get to the same circumference, or buy two.
Make sure the new one is not mounted with an old one if the axle is limited slip(LSD). It will wreck the diff. If LSD is the case buy a pair of new ones.