1991 Plymouth Voyager Transmission

I have a 1991 Voyager and I have a vibration in the front end only when accelerating, when I am coasting the vibration goes away. It only happens when I load up the van with an extra passenger. When I am alone it does not vibrate. I have replaced both motor mounts and the transmission mount. Installed new front struts and rear shocks, replaced both cv chafts. I still have the vibration it is in the body of the van I do not feel it in the steering wheel. I have checked both lower ball joints and they are tight. I checked the steering rack and there is no play in it, or the outer tie rods. The tires were balanced and the front end was aligned. I read the haynes manual and it said to adjust the motor so I adjusted it to the drivers side and to the passenger side it did not help. Do you have any ideas I am not sure where to go from here.