Rear end backlash issue on 99 Ram?

For those who know rears haha...I had the rear end rebuilt in my 99 dodge ram back in June, new 391 posi unit, bearings etc. I noticed that when I put the truck in reverse I get a clanking sound from the rear, it will also do it sometimes when I driving at a speed of 25 to 30 miles an hour then let off the gas "clank" Is that backlash from the rear or could it be coming from the trans or? Checked the U-joints and they seem tight. I took it back to the guy who did the work and we opened up the rea cover and he showed me that the gears were in specs specified, up to 6 or 7 thousands I think he said. Hmmm? I appreciate any help or advice. I might have to explain more here if more questions are needed asked?

Thanks

I'm sure the shop would have checked it but check the pinion nut for looseness, there's a TSB out on this.

Does it do it worse coasting down hill?
My 2005 Cummins Dually does is when I let of the gas slightly while rolling down hill typically. It fetches up numerous times while rolling down hill. It has always irritated me and I have no idea why it happens. Not sure if it was my injectors, gears or trans.?? Will be watching for answers.

No hills to roll down here in flatland southern jersey lol...My driveway slopes down towards the street so when I put it in reverse you'll here the clank sound and sometimes if i'm driving at around 25 to 30 miles an hour then let off the gas it clanks slightly. Yes it bugs me also. New posi unit was installed along with new bearings etc...hmmm

Yes i'll have the pinion nut checked next when i get the chance. thx

You could put the rear on jackstands and see how the rear yoke feels with the trans in PARK and then in neutral. There should be very little movement. I don't see why reverse would produce more slack. Transmission mount bad maybe?

If you had enough backlash to cause a clanking noise, you would have way more noise driving due to poor tooth contact pattern. High idle, drive shaft resonance, suspension slop are more likely to cause your noise.
Doug