When driving truck makes a chugging sound gets louder when I let off of the accelerator

Hi and thanks for using 2CarPros.

First, make sure the fluid in the transfer case is clean and full.

Next, I would suggest checking are the universal joints on both front and rear drive shafts. Here is a link that shows how it is done:

https://www.2carpros.com/articles/how-u-joints-work

If you find one of the joints is bad, here are directions for replacement.

See picture 1

DISASSEMBLE
1. Place driveshaft in a vise, using care to avoid damaging it.
2. Remove bearing retaining snap rings. Some universal joints use injected nylon retainers in place of snap rings. During servicing, the snap rings supplied with the replacement universal joint assembly must be used.
3. Position tool on shaft and press bearing out of yoke. If bearing cannot be pressed all the way out, remove it using vise grips or channel lock pliers or strike center yoke with hammer. Mark yoke and shaft to ensure they will be reassembled in their same relative positions.
4. Reposition tool so that it presses on the spider in order to press other bearing from opposite side of flange.
5. If used, remove flange from spider.

ASSEMBLE
1. Start new bearing into yoke, then position spider into yoke and press bearing until it is 1/4 inch below surface.
2. Remove tool and install a new snap ring.
3. Start new bearing in opposite side of yoke, then install tool and press on bearing until opposite bearing contacts snap ring.
4. Remove tool and install remaining snap ring.

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Start with this and let me know if it helps.

Take care,
Joe

Welcome back:
Is it possible for you to record the noise and upload it for me to hear?

Let me know.
Joe