I think i need a new harmonic balancer, but at idle and around 1200-1400rpms the whole truck shakes, at first i thought motor mounts, then tranny mount or something involved with the tranny, then that i might be misfiring because i just change plugs. not sure, but can a harmonic blanacer, when bad, make a truck shake that bad?

 

You may want to pull all the belts off the engine and then start it. This way you can see the harmonic blanacer run. Or if you have no vibrations it could be one of the other pullies.