TStewart,
As mentioned, a counterweight on the 3-PT or rear hitch might help........
You did not indicate what type of tire you currently had on the front, but it sounds like you might have a 3-rib Ag Front tire.. these have a higher center rib and lower outer ribs... they do cut into the ground because they are designed for turning in plowed ground or other field conditions.. less than desireable on dry, hard flat surfaces due to the harder rubber, higher air pressure and lack of surface contact...
If that is the case, there is a tire I know of that may help you in your situation. It is the Firestone "Rib Implement" design and does come in a 5.00 X 15 size and will fit on, I think both a 3" or 4" wheel width....definitely a 3 inch size.
Go to
www.firestoneag.com
Click through the tire product options under "Ag", then "Implement" and you should find it... It will give you all the spec's on the tire, dimension's, load limit's etc..... This tire has a wider foot print and more surface area due to 5 smaller and narrower ribs and should provide a wider stance than the 3-rib Ag. Again, depends on what you currently have however, this should help with your problem..
This site then also has a "dealer locator" search function and you can find a source near you. This tire probably has a fairly limited use and it is doubtful they will have on-hand or in stock, so they will more than likely need to order it in for you.....they are only around $40.00 or so per tire. I have Firestone's on both the front and rear of my tractor and I am very pleased with their durability and performance...
Obviously, finding a wider wheel and tire might be the best option... I am somewhat surprised that Kubota cannot help you with that since the wheel lug patterns should be somewhat standardized...
Absent that, take a look at the Firestone Tire. Good luck, let us know how you come out !!!
Add:
Also, you might want to measure the wheel diameter.. you may be able to handle a wider tire size than the one I mentioned above, as there are other options in the 15 inch wheel size in other implement tire designs but require 5 inch to 8 inch wheel width's... they then can be 6.50 - 9.50 in tire width which will give you the wider stance you are looking for without having to change wheels...... again, the tire shop should be able to recommend the widest tire possible on the wheel you have...