[man4mopar]

What's the best way to locate that? I know u can use starter fluid on gas engines. But I don't know if that would be safe on a diesel?

To answer the question ether starting fluid or water can be used. Ether will speed rev up and water will rev down.

But unless that fuel injection pump is controlled by vacuum readings I do not think that is your issue. And looking here in multiple places I do not see a vacuum read to adjust fuel metering. http://jdpc.deere.com/jdpc/servlet/com.deere.u90490.partscatalog.view.servlets.HomePageServlet_Alt

I would one at a time loosen the injector line at the injector and listen for a change in rpm. Be careful as the fuel can come out in a high pressure stream that can cause personal harm. If you hear no difference on one from the others then that is the cylinder at issue. Be it a issue with injector pump, injector and engine internals.