it'll come off... but sometimes you have to sacrifice the plug wire....
try a pair of pliers on the boot, and get the boot to spin on the plug insulator. If it does come loose, then you can just pull it off, but even that's stiff to do sometimes.
You can get a new set of wires fairly cheap for the van, even if you don't replace all of them.
Since you're doing plugs and I'm assuming this is a '96 or later.., I'm not sure if you know yet, but access to the rear #2 & #4 plugs is from the underneath of the van, up through the exhaust cavity with an extension or two and a swivel socket. Replace these plugs using an old spark plug wire boot stuffed into a smaller socket so you can get them sarted back in the holes, then switch to the regular spark plug socket to tighten them ALWAYS COAT THE SPARK PLUG THREADS WITH ANTI-SIEZE before installation(permatex is best).
Use silicone grease around and in the hole of the spark plug wires when you reassemble. It'll make it much easier to do the next time.
good luck,
Slantsixness