I think I blew a rod or something

(1998 GMC 2500, 5.7L, 4x4)I have a major problem. I was towing a trailer up a big hill. The engine kicked down a gear and hit 3000 rpms. Kept going it kicked down again and hit 4000 rpms. Before i let off the gas because i didnt want to blow it up, the engine started knocking/Tapping very load. It sounded like a machine gun going off under the hood. I was thinking i blew a rod, but the truck still ran. I lost a lot of power and it feels like im running on 7 cyl. The noise went away, but the truck sounds like it wants to stall at low idel and shakes bad. Starting off, not much power, sounds and feels like maybe a mount broke. At higher speeds, its fine. Starts up everytime, no oil leaks. What happened? I was told maybe it was a lifter and it offest a rod. Im in the middle of changing the plugs and wires. I have one more to do in the morning. The old plugs had little wear on them and looked good. Any help would be great.

1. Pull plugs back out and do a compression test.
2. Pull valve covers and compare rocker movement at each cylinder.

This is just a start to find what cylinder is the problem. Could have broken something in the valve train, maybe a valve spring, that allowed a valve to hit the top of a piston?? And the list just goes on from there.