You possibly have broken shear bolt. Pto spins bushhog is not. See if it’s broken the shear bolt. If not could be damaged gear box unlikely but not impossible. Look for the obvious first.
Any noise when the pto is engaged? We had a 350 international that did that turned out it took out the splines in the clutch that ran the pto but it made a rattling noise when the pto was engaged had to split the tractor but wasn't too bad of a job.