2000 chevy unknown driveline noise?? help

[bigbeardedbastard]

ok my daily driver is a 2000 chevy 1500 ext cab Z71 4x4 with 180k on the odometer. in the past few days i have noticed a new noise while driving. thinking it was a bad u-joint i changed all the u-joints yet it still exists. i had the 4l60 rebuilt about 5k miles ago by a reputable shop. the noise isnt followed by a vibration that i can notice but i cant pin point it as it seems to echo thru the aluminum driveshaft. i cant tell if it is coming from the t-case or the rear end? while coasting you can hear it but under power it gets louder? the rear wheels seem to be fine bearing wise?? but i am lost as far as my next move. time and $$ shortage are an issue so any help would be greatly appreciated so i can resolve this myself.

thanx
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