The quick answer is not very easily! The rear plugs on your Montana are very difficult to replace. Working from the underside is probably your best bet. You may need to disconnect the exhaust from the rear manifold to allow more room. I would recommend that you also replace the spark plug wires while you are there. Two reasons. After the plugs and wire have been together for 100,000 miles it can be very difficult to get the plug wires off without damaging them. And #2 you don't want to go back there any time soon which would be the case if you left the old wires on and one of them failed. Now you ask "Why did they make them so hard to get to". A lot of manufactures have gone to 100,000 mile replacement intervals on there spark plugs and therefore are not as concerned with access as they used to be.
ive only done this once before but i found that if you unbolt the front motor mount on top of the engine and pulled the engine forward, this gives you a little more room and you dont really need to use a lift or unbolt the exaust.attach a small chain about 2 feet long or so the lift bracket on top of the engine, then have someone help you pull the engine forward,then while keeping tension ,bolt the chain to the top of the rad support,make sure its secure because you dont want the engine to fall back in place while your arms are behind it.This worked for me,keep in mind it doesnt make it super easy, just easyier....let me know how it went!!!
Remove one bolt from each upper motor mount, pull them up out of the way then instead of "pulling the motor forward" all you have to do is push or pull the vehicle forward or back (to get the motor to move in the correct direction) and then block the wheels to hold it in place. If you are on a level surface this should take only your body weight to move the vehicle. The transmission should be in park. I have also used this to change the plugs on a '88 Grand Prix. At least on the GP this worked great!
Be careful if you try greasy1's trick. Make doubly sure the chain is secure. There is also a special tool you can purchase which bolts in place of the upper engine mount for pulling the engine forward. Also remember, when pulling the engine forward the transmission must be in neutral.