Thanks for that info outoftractluck. I am going ahead with the wheel spacer project. I've order a set of 2" spacers from Bro-Tek, which also was about $300. Bro-Tek spacers simply look a lot more professionally made than any others that I've seen on the internet.
I am taking a shortcut though - and that is having my local Kubota dealer install them for me. I don't think I have enough grunt to handle a 500# tire, even with a dolly. Cost for that is only about $50 though.
Too bad you don't have the adjustable rims. Others mentioned the extra stress on the bearings from spacers. It's there, but not a real issue (with Kubota's anyway) as the same rear is used with rims that are adjustable further out than your spacers.
I've moved many a loaded tire around. Never EVER never would I use a dolly. Do it on smooth cement or dead flat grass. You can easily walk the tire around. Walk one out, roll it aside to lean against a wall or the tractor front, install the spacer, roll it back, walk it in place. You can rotate the axle or even pull up and spin the loaded tire pretty easy. The only real issue is mental, you have to know when to abandon the tire and let it fall over , 'cuz even a superhero would be seriously injured letting one fall on him! That's just so you know.
The $50 isn't a bad price at all, but should you ever need to do it yourself, it's a 30-40 minute job for me to pull both filled rears off and swap sides (needed to adjust rims to centers for some widths). Never dropped one yet (knocking wood like mad)...