I have a 2005 Chevy Blazer 4 WD, 5 speed manual transmission... there are noises that sound "like metal that is being stressed" coming from the rear driveshaft. I looked at the driveshaft and turned it under my own power and I could hear the same noise.
To be specific, the noise I hear is more prominent when I am just taking off in 1st and 2nd gear (under the most torque I guess is the best way to describe it).

When I am in reverse I not only hear the stress noise but also a whining as each revolution of the driveshaft goes by. If I take it out of gear and let it coast in reverse the whining noise goes away.

Any idea what this is and what I need to do to fix it?

Thanks.

. Check for an intermediate driveshaft bearing/mount--the bearing may be shot.

. Check the gear oil in the differential--it may be low or it may need to be drained and replaced.

. Check universal joints--they may need replacing. Ensure mounting bracket bolts are tight.