Couple of things to consider with an extended warranty: they typically are an extended warranty, meaning they kick in only after the regular warranty expires. If the manufacturer's warranty is 36k, a 100k warranty is actually a 64k warranty. The extended warranty often excludes a lot. Read the fine print. The GM warranty isn't affected by the PTO, but an aftermarket warranty might be.
We don't buy extended warranties. They wouldn't pay off for us. The only routine problem we've had with GMC vans is the "Hydroboost" that provides power assist to the brakes and steering and having the paint fall off at about the seven year mark.
The last vans we bought (October 2013) had all of the extra coolers we usually buy already installed as stock items, so in the chilly hilly you live in additional coolers probably aren't needed.
The last time I bought a new car, the extended warranty offer started at $2,500, dropped to $1,900, then $1,500, then $800 before the dealer gave up. He admitted that the warranty cost them $500.