What can cause an intermittent, torque and speed sensitive knocking from the back portion of the drivetrain?

On two months separated occasions my AWD Durango has made a rather
noticeable "clanking" or knocking sound coming from behind the front seats -
but only during moderate to heavy acceleration.

I know it must be drive-line related because the frequency of the sound
increases with speed - the faster I go the shorter the duration between
clanks/knocks.  It does not seem to be more noticeable or enhanced by
changes of direction (like going around a left vs right turn, etc).

5.7 HEMI, AWD

Of course it hasn't done it when I've had it in to a dealer or independent
repair shot.

Interestingly enough, my son has a 2001 Audi A4 1.8T Quattro that is having
a very similar problem.

I will try to get a recording of the noise in my Durango but have one from my
son - his is definitely a KNOCK where mine is more of a clink or clank.

Any productive thoughts will certainly be appreciated.