U joint? Tiny guttural rattling sound when first accelerating

Don't count out the exhaust...not hearing it of course doesn't help my post.

The exhaust can spin from a weak connection behind the converter and make odd noises.
The heatshield can contort and give a rattle as well.

A rear u-joint is fairly common on these. Symptoms can range depending on the degree of deterioration.

When they first start going bad the become stiff and tend to chirp or squeak most noticible at slower speeds.

The needle bearings in the caps begin to rust and swell at this point. As they begin to powderize the joint begins to have play or moves out of center causing a vibration or a klunk when put in gear.

Look for any rusty looking powder around the caps and check it for play.

Also these trucks have had issues with the right rear parking brake cable. It may be partially stuck in the engaged position Drive a block or so and feel both drum for a significant temperature difference.

Check the fluid in the rear diff and make sure it is adequate.