Weird Vibration!

Ok guys, as some of you may or may not know, I have finished My duster. (are you really ever finished though?)

anyways its a 1970 440 4sp 8 3/4 sure grip. What is happening is if I am cruising down the RD and I put it in nutral, It has a vibration that comes from under the car, You have to be rolling at around 30 miles or more.

The two big parts that I did buy used and put on this car was the 4 spd transmision, and the 323 sure grip pumpkin. I am guessing I have a bad bearing in one of the two, But where and what?

If I am parked and I rev the engine or hold it at an idle it won't do the vibration, regardless of the rpm's I hold it at. It only happens if the car is rolling in newtral, i think I can sometimes feal it when It is in gear two, but I am not sure. You can feal the vibration through your feet on the floor boards near the midle/back of the car ( under front seats, or where you put your feet if you are sitting in the back.

Parts that are NEW are...
Both U joints front and back
axles
axle seals
axle bearings
all the rear brakes
Clutch
preasure plate

I have been told to put the car up on jake stands and drive it with the wheels off the ground and see if I can get it to vibrate. but I won't have time to do that for a couple days, so I thought I would see what you all thought.

I might not be on the computer to answer questions right away either.

Thanks ahead of time. I am sure someone will know what it is :cheers:

Phil