Like he said ^^^ check the oil for water or inside the oil fill cap for a white residue. From moisture being in the engine/crankcase.
How long have you owned the truck?? The reason I ask is because if it's a recent purchase and had a prior head gasket problem, or an annoying leak someone may have put Bars Leak (which if isn't dissolved completely can leave gunky chunks) Or another sealant in there which may be the residue. So don't jump the gun just yet.
Worst case scenario: You may have to do major engine work.
Alternative: Pull the radiator and drain the oil.
Check the oil for a milky consistency or send it out to have it checked. Although you mentioned you could see the leak, you didn't mention where it was. I'm going to venture a guess and say that the seam on the end cap blew apart which is a common problem.
If the oil is okay and you can get away with just the radiator...don't forget to check the transmission. Another I didn't notice was if it's a manual or auto trans.
Auto: What cools an automatic trans?? The radiator. It's nowhere out of the question that the trans cooler in the rad may have gone bad and contaminated the coolant. Which brings me back one giant step...If the engine was overheating or leaking.
There was this one time at band camp that our bus overheated and since the radiator got hot...the trans got hot too and needed to be checked too.