Tom, You need an overrunning clutch. When I shred with my D series tractors, the over running clutch is just as important as if the tractor had no live power. You can use the power director to slow or stop in heavy going. When you need to shift, it is still gear grinding no matter how you do it. The overrunning clutch will allow you to shift.
In my experience, the only time the over running clutch is not needed when running a shredder is if the tractor has a completely independent PTO. In the 50's, Oliver had this, Cockshutt had it as well. Haven't experienced John Deere but I think They did also with the number series. Allis went a different way which I really like for most applications - even shredding. But the Allis needs an over running clutch.