Nah, not the mud flaps, the sheilding under the engine. Or it could be anything in the engine bay that vibrates once the revs get high enough. Next time it vibrates, see what revs its doing, then try to do the same revs in a different gear, or even while parked, and see what happens.
Also, Injector cleaner will only clean injectors. See your mechanic and he will put som de-carbonising stuff into each cylinder.