5.9 Magnum help needed

If its not one thing its another right? Well I drove the truck about 40 miles today without a problem. Once I get back home the truck begins to shake. Then the engine light pops on. It feels like its not balanced. I checked the harmonic balancer and its fine(rubber seal in the balancer went out in my camaro and felt like this). Any ideas? The belt tensioner right under the alt shakes and moves about 10 degress while the engine is running. Is that normal?