Nasty vibration...please give your 2 cents!!!

[Dhartmann]

So ive been driving my 69 K10 for about a week back and forth to work and ive been noticing a really bad Vibration around 55-62 mph. I wouldnt describe it as a 'shake' its definately more of a vibration. Its very noticable on the floor and through the gas pedal. Its really making me nervous because it honeslty feels like my wheels are going to fall off or the trans is shaking the bell housing bolts loose..Pleasee give me any feedback you might have. ---The wheels are all tight, bell housing is tight, drivelines and u joints are tight. It only does it at 55-62 though?? Could a driveline me out of balance..trans mounts..?? Help!! Oh and one more thing..when i put it in park and rev the engine up to 4k there is no vibration...so rule out the motor causing the prob. Thanks!!