Thanks for your question.
When you said "hit hard once" do you mean while cranking, the engine locked up momentarily?
How miles on truck?
What year was this truck?
From what I'm reading, it sounds very likely that the head gaskets have failed allowing coolant directly into the cylinder. This is why it locked up when cranking. It hydralic locked, that is not good either, because if that happens enough times, it could bend a connecting rod.
You might not see any coolant in the oil. If the coolant is going right into the cylinder, it has a very hard time getting into the oil because of the piston rings.
You should be seeing at least a slight white smoke from the exhaust and a sweet smell from the exhaust. This is confirmation of head gaskets bad and need to be replaced.
If I suspect a bent rod, when I get the cylider heads off, I bring all of the pistons up to TDC and compare how high all the others come up to the top of the cylinder. If I notice one of the pistons isn't quite at the top like the others are, more than likely the rod is bent on the one that is a little short. Also look at all of the cylinder walls for scoring. Bring each piston all the way down, and look really close at the cylnder walls. If there is excessive scoring, that means something is wrong, and you would need to dig in deeper to find out what has caused this.
Also it wouldn't hurt to check the oil pressure too before tear down. You want to make sure the oil pressure is sufficient.