I recently bought a 1996 Chevy Suburban 1500 4x4 with 18K on it, and it is one of the best vehicles I've ever had!
When I let off the accelerator at low speeds (10-20mph) and coast, and then apply the throttle, there is a clunking noise from the limited slip differential. I can feel what seems to be drivetrain slack. The dealer, of course, claims this is normal and due to the limited slip.
A 4x4 driveline includes a lot of parts between the engine and the rear wheels. These parts are assembled to make up the transmission, transfer case, drive shaft and rear axle. Unfortunately, all these items are not welded together. That's why there's a necessity for specified "clearances" along the way. When you decelerate, all this slack is taken up in one direction, and then upon acceleration the slack is taken up in the opposite direction. This can produce a slight and, yes, normal clunk.
Excessive backlash between the ring and pinion gears or various worn out items within the rear axle can produce unreasonable clanking. However, the limited slip aspect of the differential would have little to do with this type noise. A clicking on turns would be another story.
Also, the problem may be way up front, but due to a "funky" audio transfer principal it sounds like it's coming from the rear.
If you truly feel the noise is beyond normal and the technician is missing something, then ask to go for a ride in a similar vehicle and compare clunks for yourself. If your clunk is noticeably louder than the other clunk then there's more work to be done. On the other hand, if your truck sounds just like the test-truck, it's probably a normal characteristic.