Hello and welcome to JustAnswer!
There are a couple of ways you could go about blocking the rear evaporator.
If you look underneath the van on the right side at about the front of the passenger door area you will see the two large aluminum a/c lines running to the rear, and in this area there will be a brass fitting where the lines to the rear join the lines that come down from the firewall. You could take them apart at these fittings and install some caps on the lines. I would recommend capping both sides of the lines so you didn't get any debris or water in the rear lines while driving.
You could also remove the lines from the rear expansion valve where the rear lines go up into the body. If you remove these lines and have a blockoff plate made you could bolt the lines to the plate and seal them to it with the O rings that are already on the lines.
I've heard this problem is fairly common on this model year and a couple years after. Do you have a ballpark estimate as to how much a fix costs at your typical Chrysler dealer?
It's not as common on the 2005. The most problems are on the 2006 and 2007 vans, which use a bit of a different design.
The labor time to replace the rear evaporator is 2.7 hours. The part itself runs $290 from a dealer, it would be less from an aftermarket source.
The price I mentioned wouldn't include refrigerant. You would be billed for three pounds. At my dealer R134a runs about $12 a pound.
Have you tried any parts stores to find an aftermarket evaporator? If you've just tried online you might be able to find something through a parts store. I can't say for sure if one would be available or not, I've never looked for one for one of these vans.
I looked the evaporator up again and it's actually $349 from a dealer for a 2005 van. You can get a Mopar one HERE for $240.
No, unfortunately I don't know what size they are. I've never had to look for a cap and they don't give dimensions like that in the service or parts manuals. They may be a metric thread if it's a size in between two American sizes.
Good luck with it!