E-350 overheating

I have a 1998 ford e-350 5.4 triton. It is overheating, mainly when going up hills. It does not overheat when driving on staight aways or down hill. It also does not overheat when sitting idle. I have removed the thermostat and it still overheats when going up hills. I see no leaks anywhwere. Can you help?

If it overheats with the thermostat out the radiator could be plugged up. Do this. Start it and feel the upper radiator hose. With the stat out you should feel the coolant surging through the hose. If not the radiator is plugged up. When the temperature gauge reads hot after starting and running the motor, reach down and feel the lower radiator hose. It should feel warm. If it feels cool to the touch, the radiator is plugged up.

Ok, I tried your test. I could feel the water flowing through the top hose and the bottom hose was hot. Could I have a bad water pump even though it doesnt leak? Also could it be the fan clutch? I dont want to dump a ton of money in this if I dont have to. However this is my work truck and I need it to run good.

I read in another thread how to check the clutch motor. I did so and mine seems stiff. Its hard to get at with the shroud in the way but I can barely get it to make one complete rotation in either direction. I am really stumped here.

I changed the Rad and it fixed the problem.