I am positive it's not suspension related.
Here is when it happens:

Accelerating up to a certain speed usally higher speed (over 60km/h) and then when I reach crusing speed and the instant I stop accelerating (the trans shifts to a higher gear) I get a noise coming from the rear.

It's not a whine or high pitched but a low gutural sound for 2 secs or so.

Any thoughts?

thanks

 

Can you make it do it at any time while at road speed by completely removing torque from the drive line? i.e., not in "drive" or "coast", but in "float" mode?

If so, it's caused by play somewhere in the drive line; could be the rear end, or the U-joints.

Try replacing the U-joints.... if you haven't done that, they're shot like OE ones always are. If it still does it with good U-joints, it's probably the rear end; specifically, loose pinion bearings. Sometimes you can get it to work for a little while longer by tightening up the big nut, but that's risky if not done with the carrier out of the rear.