My truck does that too. Very light clunk, dealer refuses to investigate. Took mechanic with me and everything, went to a parking lot and did a few "forward reverse" cycles. Go forward, brake, stop completely (but for < 1 second) and switch to reverse, then bit of gas... clunk. Like clockwork. Mechanic said "you're shifting to reverse too fast, wait for 3 or more seconds and try again". So I did that, waited for three seconds, and then there was no clunk.
But, I told the guy it does it when downshifting coming up to a stop light as well, if the light is red, and I roll up to it and turn right, sometimes I don't stop 100% and hit the gas if no traffic is coming, and then again, clunk.
Of course he wrote up his notes only that I was shifting into reverse and hitting the gas too quickly, so now on his official notes it looks like user error and that I'm abusing my truck. I was completely pissed when I saw those notes, dude completely ignored my comments about the stop light or four way stop sign, which, is never user abuse. I should be able to be rolling at any speed and hit the gas without an audible clunk. But now if I have a warranty issue and Ram sees those mechanics notes, guess what, "user abused his truck, we're not going to pay out." And never mind that in my testing, I always stop 100% before switching, and give it a quick wait, I never do rolling gear changes. I shouldn't have to wait 5 seconds though, that's complete BS.
Going to take it back again once I'm certain I can reproduce it 100% without having to shift gears to make the sound appear.
I'm not sure what it is, maybe something in the drive train or maybe even a suspension issue, hitting the gas is when the truck rocks in the the opposite direction of my current gear.
Like you, mine has done that since the first 3 weeks of ownership. There is definitely nothing I could be doing wrong (in terms of abuse) that would make the truck clunk that quickly. Many mechanics these days just plain suck (or there is policy in place preventing them from fixing "minor" issues).