the tires have about 25k on them
I replace all tires at once no matter what vehicle is involved, but that's more paranoia than anything. I would at least replace both front or both rear tires at once. AWD compensates based on grip so certain discrepancies in tread shouldn't be that big of a deal.
No need to change all 4 if the other are good. I would not advise a new one on the same axle as an old 25K tyre so check your spare and dump the worst of the 5 and put 2 new ones on the front.
On an AWD vehicle mismatched tires can overheat the transfer case causing very expensive damage. If the existing tires are more than 20% worn then yes, absolutely replace all 4 at once. Most tire shops will insist on this anyway since they are not eager to pay for your new transfer case. On some models if there is less than 20% wear it is OK to replace a pair so long as the new tires are identical in terms of size, brand and model to the two tires being kept. Consult your dealer or the vehicle manufacturer to be sure. PS Yes, I have actually witnessed AWD systems being damaged by mismatched replacement tires. The last one (about a year ago, 1 tire put on by another shop, not mine) the car lasted about 30 minutes after the new tire was put on before the transfer case gave out. I believe the lawsuit is still pending.
