Approximately 1996 model runs all day long at low <2000 rpm but when I idle up to engage the PTO it immediately overheats. Thermostat / radiator / other. I am new at this and any help is welcomed.
How have you determined t's overheating? If it's the gauge, have you checked it with a thermometer?
Make sure the thermostat is opeing, the radiator air side isn't clogged with chaff, the water pump drive belt is tight and not slipping, the air cleaner is not clogged, the rad cap is the right pressure rating and is working, the coolant side of the radiator isn't clogged. Look for bubbles in the coolant indicating a head gasket leak.