My truck makes this weird noise when accelerating as well as while under load going down the highway. I thought that it was the centramatic's that I put on about 6 months ago. But when I rotated the tires today and removed them, it still does it. It's in between a squeak and a rattle. It gets faster as I drive faster and shifting does not affect it other than in between shifts it stops and once load is put back on it starts again. It does not do this all the time, but it does do it more than it doesn't. When I had the truck on the lift today I checked driveshaft and bearing play but it did not seem to have excessive amounts anywhere. The truck has 141,000 miles on it and I just drove 2,500 miles last month from Washington to Mississippi and it did make the noise periodically as well. I greased all the zerts before I left and changed the rear diff fluid too. What do bad bearings sound like if they're starting to go out? I don't think it's them, but am willing to replace them if experianced people have heard of this situation before or think it the best route. I have also checked for loose brackets and found none. I have put a new set of 4.88's in the rear that ZUK set up for me in the last 8 months as well, but don't think the problem is in there. Sorry it's so long, but wanted to provide as much info as possible. Thanks.

p.s. I did search, but couldn't find anything relavant.

Here's my $.02 (might only be worth that) I had a noise with similar behavior. The u-joints seemed perfectly fine when examined, but when I replaced them (on a mechanic's recommendation), the noise went away. You might try running the truck in 4wd without the rear driveshaft and see if it still makes the sound.

my wheel bearing went bad on the driverside rear last year, it was a constant sound while in motion, increased with speed, kind of sounded like a subtle grinding noise behind me, i went to a shop and jacked it up in the rear, but it in gear and let the tires spin idling in drive, a trusting friend can listen near the backing plate which will tell you if that's the problem.