I'm not expert on auto trannys, but it sounds like you may have a slipping gear clutch. check the fluid level and add if necessary. My Jeep was exhibiting similar problems and it turned out to have a leak and all the fluid would drain out in a few days time.
Good point huss.
To test if its a driveshaft problem is easy. Accelerate your car until the vibration is worst, then put it in neutral. If the vibration ceases its the tranny, if it doesn't its likely the driveshaft/u-joints.