Can you shift gears with the engine running? if the transmission stops using the clutch then the PTO should also be stopped. If both are moving and you grind gears then yes it sounds like a clutch issue. Not sure if there is an adjustment.
this may or may not be a help to you, but one way I tell if the PTO clutch is releasing or stuck is this: With the tractor switched off and the key in my pocket, I take a long pto shaft and hook it to the PTO on the tractor. I then support it so it is level and square with the tractor. I then wire the clutch pedal to the running board down as far as it will go with the pto lever engaged. next try turning the pto shaft out on the end with a bar or something that will give you leverage. I have freed up stuck pto clutches like that, and sometimes that don"t work either. I wouldn"t put a lot of power in trying to turn it like this. You may get hurt or damage something. The other way is to place a pipewrench or bar,(something about 3 or 4 foot long) At an angle like say 10:00 position,on the end of the same long pto shaft and hang a big weight on the end of the wrench/bar. then go depress the clutch slowly and see if it will release, dropping the heavy weight. (Keep the kids and dogs away from all this!)This may not help you out, but it gives you more information to research your problem.
Or you visit youtube for video guidance... I would also recommend good manuals but for now if you try this and it doesn;t work then we'd switch to manuals. Good Luck