You might get lucky, and it might free up. You could lift the front wheels off the ground and jiggle things back and forth. Alternatively, you could remove the front driveshaft for a while and see if things "unwind". Or you could get lucky, and the linkage is just stiff. My 4wd lever is stiff to engage/disengage. I have to remind myself that it takes a sharp snap to do the deed.
Let us know how things turn out.
PS; It might be good to look at another
L3600 GST to see how it feels to disengage the 4wd, how much force is required, and the length of travel. BTW that GST is a cool transmission.
PSS: You might get some advice on TBN from Kubota owners.
PSSS: It would be good if you could talk with the previous owner either before or after the purchase.