Trans or motor mounts problem??

When im driving in D and just cruisin like at 30-40 when the car has already seemed to shift in its top gear, i get a vibration from under the car, kinda bad, but as soon as i hit the gas a little harder and bring up the rpm's it goes away, only does it when just cruisin barely hittin the gas, i cant tell if its a trans problem internally, or a trans mount, or even a motor mount??, how many trans mounts are there? it just seems like something is loose and vibrating when the car is in its top gear or any gear and not accelerating very hard. its the 4.6 v-8 , auto trans by the way.

Sounds like a torque converter shudder to me, I'd change the tranny fluid, filter and drain the torque convertor. Refill with good tranny fluid.