Noise when accelerating at certain speeds - rattling/growling near front

[centuryhouse]

I have a 72 GMC longbed w 350 engine and turbo 350 trans, two piece drive shaft.

I took it to work today and a new noise is present when accelerating, but only a certain speeds - around 5-10 mph, and around 30 mph. So that is in two different gears - first and third (?).

The noise is kind of a roaring, growling, almost rattling sound - kind of a cross between how power steering sounds when it is low on fluid, and how a diesel truck sounds. It does NOT seem to be from the engine.

* The noise stops immediately I pop it into neutral.
* The noise also goes away in gear once I pass the speeds in question, and the truck drives and feels normal.
* The noise is NOT replicated if I give it gas while in gear with my foot on the brake.

I thought wheel bearing, but it stops when I pop the trans into neutral, so not that.

I have replaced the two u-joints on the rear drive shaft, and replaced that rear yoke at the carrier bearing. I have NOT replaced the carrier bearing or front u-joint at the transmission.

Could it be the u-joint or carrier bearing?

If not, is there something in the transmission that could do that?