Howdy sano888, welcome to the tractor forum.
I've read that the Kubota L35's also have a problem with the rear wheel bolts coming loose. The owner's manual recommends torqueing them every 25 hours to 160 ft lbs.
One solution that I've read about is to replace all the troublesome bolts with studs, and use red loctite in the axles. This apparently was successful. Your Kubota dealer should be aware of this practice. Discuss with your dealer.