[wrench459]

Sounds like you've got two different problems at the same time.
When your taking off from a red light under heavy throttle..does it sounds like someone under the floor pan with a hammer hitting maybe three or four times? If so look at the motor,trans, or torque tube mounts.
The u-joints especially the front can cause vibrations thats speed related.
They'll be in tune at certain rpm's.. first gear vibs at 10 mph second gear vibs at 20 mph for example.
The front pinion bgr. when going bad is a constant noise at all speeds.
Axle bgrs. tend to make more noise when you shift the load from one side to another..trust your ears.
Then theres coast and decel noises from the ring and pinion..
oh I've heard the 69-70 axle bgrs are almost impossible to find.