My Challenger rides smooth as can be. Only vibration is from my teeth when the elephant wakes up at a stop sign. But my car's totally redone. My bro's 69 Charger drives like a dream & the ride in his 68 GTX was perfect, too. But those cars were totally restored, too. Be sure to check your steering box/front suspension, bushings, spindles, brakes & tires old or out of balance. One wheel may be tweaked from hitting a curb? As was said, there's a lot that can cause vibration including tail shaft bushings, trans mounts, u joints, rear end/axles, drive shaft...
I'd start w/ the tires, wheels, ball joints & bushings first.
Good luck.