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.