How can the lower arm get jammed against the wheel guides for the lower rack?  The arms don't extend that far....at least not in my KA. 

Gary

Sounds like the pin for the wash-arm support is loose and the whole wash-arm is floating up with the force of the water.  You likely just need a new wash-arm support kit, and maybe a new wash arm.  If the machine has had enough use for the support to be bad, the bearing in the arm is probably shot too, especially if it's been running loose for a while now.  They show up on ebay often enough and it's a very simple repair.

Hi you need a new or good used wash arm support and probably another wash arm, the shaft should not be loose at all ,it is not removable.

I can understand your frustration, as you have already made repairs to this machine.   If you are done fixing it and want to replace it, be advised that modern dishwashers will not clean as well and will take three times as long to get the inferior job done.  You might want to cruise Craigslist for a later model KitchenAid ranging from series 21 through 23.  Whirlpool took over this division from Hobart starting with series 21 and immediately made improvements to the detergent dispenser system that made it less prone to trouble.  The In-Sink-Erator brand dishwashers from this period are KitchenAid clones, so they are an option for you as well.

If you like the way your KDI-18 has performed for you up until this latest issue presented itself (and there are legions here who will tell you the 18 series was the best KitchenAid dishwasher ever), I can assure you that you'll be disappointed if you replace it with a brand new machine, even if it happens to be a KitchenAid.