Mikes51Chevy: You did not say if you had any trans or engine work done to your truck. You stated that you can feel and hear the noise, and at about 30mph, out of gear It's still there.
1. Check transmission for tightness to bellhousing
2. Check trans oil level, pressure plate tight?
3. Check driveline, timed and u-bolts tight.
4. Check rear end oil level. Check tires/flat spots, or blisters, missing tread.
5. Check pinion for looseness or leaking oil from rear. Possible loose rear gears or bearing noises and/or vibrations. Maybe a loose rear wheel. More likely worn axle bearing, or pinion. You said the noise is apparent when no torque is to rear, that points to rear gears (ring loose or pinion worn),
worn bearings and things moving (thrust) around.
6. An easy check is to support truck frame on jack stands, or blocks to frame. Remove rear wheel/tires and place a floor jack under rear end
differential. Also block front wheels both ways.
Start truck and check for noises, easy to locate with wheels off. Rev truck up to about 45, and let off. If you can't tell, rev to 45 then shut off. By careful listening and checking you will be able to pinpoint the trouble. Good Luck!
Think Chevy!

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