Try placing your right foot on the treadle pedal and rock it back and forth (moving the machine slightly forward, then slightly reversed) while placing the heel of your left foot of the 4wd lever forcing it down. You should reach a point where there is no binding on the shift collar and it should release into 2wd.
When you have it in 2wd on a flat surface, how easy does it shift between 2WD and 4WD without moving the machine? If it's still problematic, then you got more going on beside a lag-lead match.
What size and type tiers to you have on the front and rear? See attached for what Kubota would have available from the factory.