Not being a technical guy, I'm not sure what the shaft is called but on that truck there was a short (less than 16") shaft (input shaft maybe?) that came out of the transmission to a U-joint that was connected to the driveshaft. This shaft was round, like a cylinder, but one side of it had worn a flat spot on it. So, it was no longer perfectly round. With each revolution, their was a vibration and rumbling much like you are describing. With faster speeds, it was much more noticeable...like yours.
Being that the problem you are having is a known Toyota issue, that is probably what you are experiencing, but maybe the problem I had is your problem. If so, it cost about $35 to have the shaft remade and I did the work myself so it is much cheaper than a new differential.
Hope this helps. Let me know if I might be able to provide more info.
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