Couple ideas.
Contact the State AG office.
I get free legal aid through my employer. I well crafted letter from a lawyer can really wake some people up. Even if you have to pay a little, it may just get their attention.
Contact the NTSB and file a case. This is clearly a safety issue. Not sure if they handle tractors, but worth a shot.
Start climbing the chain at Kubota. If someone isn't responding, find out who their boss is, contact them. Keep going. Be persistent.
Send a certified letter to Kubota requesting their official disposition on your case along with reasons. Copy the dealer on this and request their version as well.
Make sure you document every call you make, who you talk to. Make sure they know that you are recording their name and such. Eventually, they will figure out you are putting a case together.
Get a video of the problem occurring.
If you financed with Kubota, contact them.
Lastly, go to another dealer, pay to have it fixed, then ask Kubota to justify why it was not warranty.