8.25-3.21 suregrip

Typical Mopar.....
When you put the tranny into reverse, the pinion gear reverses direction on the ring gear. If there is any slop (there always is, even when new) the noise you hear is the pinion gear hitting the back side of the ring gear teeth. Jack the back of the car up, put the tranny in neutral, crawl under it and rotate the driveshaft back and forth to see what I mean.

The best way to keep the clank to a minimum......take your foot off the brake before you put it in reverse.