It could be something in the under dash assembly but chances are higher that the problem is in the tractor section.
You can tip it up on it’s bucket so the handles are up in the air. Put a towel or something to protect the paint edges.
Unbolt the bottom cover and look around in there. If someone is available have them move the shifter while you look at the parts.
The friction disk, part with a rubber edge, moves right and left. It gets driven by the aluminum drive plate in back.
The handlebar clutch lever makes the friction disk meet the drive plate.
If the friction disk is always touching the drive plate shifting will be very hard.
If grease on the hex shaft the friction disk rides on has picked friction disk dust or the grease hardened that could cause hard shifting.
If the shaft mounting bearings are worn causing slop that can make shifting hard.
You should be able to rotate the wheels with the clutch disengaged. Tape down the clutch and the friction disk should press against its drive plate making the wheels very difficult or impossible to rotate.
You should be able to rotate the friction disk by hand when the clutch is not engaged.
Look around in there and play with the shifter and the problem should show itself. If you have a gas tank shutoff close it before you tip up the machine. You might get some car leakage when you tip it up which happens on some models.
