Paul O., it sounds like you may have a sheared Spirol pin in the drive shaft to Hydro pump connection. This would be in the area under the tunnel so it's kind of hard to work on. You may have to split the tractor. The Spirol pin will most likely appear to be OK as in the two visible ends will look OK. It would be friction of the cast coupler and broken ends of the Spirol pin on the hydro input shaft that is causing it to work with a light load.
Look in this area for the Spirol pin.
Look in this area for the Spirol pin.