1996 Chevy S-10 Why is it viberating

I have a 1996 chevy S-10, 4.3l, 2wd 5spd manual, my truck has been lowered 2"-front/3"-rear. I have noticed that when driving my truck shakes as the rpms rise, it smoothes out a little when I get to higher rpms. Although when not in gear and the engine reving there is no problem. But when the auto is in gear, reguardless of speed the truck vibrates. I checked the spark plugs thinking there was a misfire, they looked great, perhaps you know whats going on?