With an AWD vehicle you are "supposed" to replace all 4 at once or if that's not possible, get a replacement (in the example of a blowout) that is as close to the size of the others as possible (tread depth).
Reason being you can mess up your drive system with different sized tires. Without going too much into it all, lets say you have a tire that's brand new tread on the rear right and you have a newerish tire on the back left but it's 7/32nds treadwear. When the rear locks up, you have the old tires now turning faster than the new tires (newer tire has a larger diameter).
Possibly even the TPMS since you'd technically have one tire spinning faster than the others, so it may indicate a different tire pressure then there actually is.
I believe it could also be bad for your antilock brake system but that would hold true for any vehicle.
In reality, some people just can't afford to replace all four tires. If I was in a pinch, and could only get two, I'd put the best used ones in the back and the two new ones in the front.