4Runner Vibrates Over 2000 RPMs While Driving, but not Parked

My 99 4Runner vibrates when the RPMs near 2000, regardless of speed. In fact, when cruising the interstate, when I ease off the gas and cruise down a hill it’s smooth and silent. But regardless of which gear I’m in, when the engine revs up a bit, the car starts vibrating. Parked in the driveway, I can rev it all I want without vibration.