IMHO,
disconnect the connection of the shifter on the gearbox
and make sure the shifting linkage works smoothly.
(no resistance in the shift stick to the gearbox)
If that's not the case, fix the kink in the linkage/shifting cable and fix that first.
then it'd be a good time to check if you can set/reset the gearbox by hand.
there will always be spring tension to overcome, but the shifting itself should be smoothly (although not effortlessly because of said spring)
if the lever itself runs smoothly and the shifting cable or linkage didn't get stuck on the way to the gearbox,
or if there is a sincere "wrong" resistance shifting the gearbox directly,
the problem will likely be the gearbox itself,
see if you can see anything being cracked or bent or otherwise out of the ordinary..
and if not it'd be time to crack it open I'm afraid.
(I think yours is integrated into engine case'S PTO section..
a few bolts to remove and the gearbox should seperate.
check that all gears and bearings are in good condition and well aligned,
regrease/oil and put it back together .
reconnect the shifting linkage and you should be good.
be carefull though, the roller bearings inside are of slightly different sizes
so take carefull notes which one is which if you remove them from their seats.
(at least three different sizes 6202, 6203 and 6204 IIRC)
if you have and external F'n'R gearbox there are two common options,
a small round can style and a slightly larger oval'ish McNugget shape
We'd need to know which one you got in order to help you with it.
but the idea is always the same, open it up, find what's causing the issue, repair/replace and put everything back together nicely
'sid