Trembles over 45MPH

Okay, I've got a problem once I reach speeds of 45 mph or over. I've checked the Tires, Rims, Replaced motor mount and transmission less than a yr ago. The problem started months after I replaced the mm & tranny. I don't feel it on the steering wheel, but the overall car.

It's a 1970 Chevelle any and all info is greatly appreciated.
Thanks
[email protected]

U-Joints

I would agree with Cruzn69. Especially if you notice it at deceleration. It is most surely your u joints.