Larry I have been down this road, we had a catastrophic failure of our Hdy. system after a failure of the main pump 06PT after replacing both pumps, cooling fan. fan blade, power steering and the Danforth wax valve (3 of these).
The wax valve may be your issue. The valve uses coolant temperature to regulate fan speed they are available in 5 degree increments from Source Engineering in Eugene, OR.
We had the major work done in AZ after the breakdown and tow to Massey Diesel ( Tom is no longer with them a big loss)
What temp are you running has the coolant temps been getting higher?
The valve is not expensive and may be a good starting point. I remember that Tom did temporarily remove the valve to check for high speed fan operation.
If you pump is really the problem you can have them rebuild it.