The 30 cycles was a SWAG from turning the thing by hand and observing the amount of vibration induced to the rest of the framework.
The differential idea is a good one, I have access to all that sort of stuff, I'll have to go scrounge around and see what I can find. A nice light one out of a compact car or pickup should work well. Thanks fer the idea!
Your PTO is 12" off the ground? That sounds awful low. Standard hitch height for a fixed hitch is 22" or thereabouts, and the PTO is usually 8-12" above that. What kind of tractor is it?