Why is truck shaking when throttle is applied but smooth when coasting?

Is it the engine that is causing the shake like an engine misfire due to a bad plug or spark plug wire or even a vacuum leak or bad fuel injector. Make sure plugs and plug wires are OK make sure air intake boot from air flow meter to intake manifold is not cracked. If its a chassis vibration look at the drive shaft support bearing and drive shaft u-joints, then look at the axle mounts and shackle bushings. It's hard to guess without being able to experience the problem first hand.