It's "play" in the driveline. Small tollerances stack up by the time power hits the tires. I've owned a LOT of trucks and it's very common in 4WDs. My Landrover Discovery was REALLY bad. I did some battle with the Rover dealer over what I thought was too much. I call it "backloading", the technical word is "backlash". The driveline wants to push against the slowing wheels, then pulls against them to move them foreward. Play in all the gears creates a thunk if pressed too hard too quick. I found that it needs to be managed well, reload the driveline before applying heavier power to it. I can go down the road in my H2 and press and release the accelerator in a rythim and really get it clunking hard (I tested to see what the backlash was when I got it) but it's not bad at all compared to some trucks I've had.
