The pump has a weep hole in the bottom where the shaft is. That is most often where a pump leaks first. Use a mirror or get under the truck with a flashlight. Old dark rust in the hole not as likely. New corrosion, wet with green coolant, white/green stains, that is likely your issue. Remember if you replace the water pump some bolts get RTV sealant as they go through and into the block area. So research or reach out to TNBrett as I recall he remembers which ones to coat. Prevents water/coolant transfer to your oil. Use SCA coolant or coolant with SCA additive. Used distilled water only. I would also do the thermostat, but only use a Motorcraft one.
Also look around the base of the upper radiator hose spout. It is also the thermostat housing, that gasket could be leaking.
Another and more common issue is perhaps your oil cooler seals are leaking. Look at the cooler for similar markings. When cool open you radiator cap and look for oil. Pull the dipstick and look for water at the top (Engine sweat.), oil mixed with water(Milky white.), or your oil level s up. Often when the seals fail oil and coolant mix.
Other potentials.
1) Coolant temp sensor.
2) Cold start advance temp sensor.
3) Radiator has a small leak.
4) Head gasket.
5) Block heater.
6) Freeze plug.
