What Charles may mean is that you are not totally dis-engaging your clutch and thus the gears are turning when you try to engage the PTO.
What is a 4 foot topper? Is this a mowing implement?
The other issue may be the implement is driving the PTO shaft and causing the gear movement.
If the PTO is disengaged when you start up and the implement is not turning. You then press the clutch with the engine on. The Transmission should not be turning and thus no grinding. In this condition what happens when you try to shift into gear? If you also hear grinding then you have a stuck clutch plate.
May just need a little persuasion to free up. A simple way... (I may be wrong here as I am pulling from my memory)
Jack up the back end and with the engine off in 4th gear and clutch pedal down turn one of the rear wheels by hand.
Let us know what happens.