100 hrs on a almost 3 yr old tractor!!??? You need more seat time
One thing you might try is swapping tires left front to right front and the same on the other side and that should tell you if it is in the tires or drivetrain. Tires I would not worry, drivetrain I would get checked out. This tractor sits ALOT it is possible to have flat spots develop on the tires. It could be more noticible in
4wd because the front tires are pulling. I always place my motorcycle on a lift for the winter because of this.