If it's in 4-auto only, my guess is it's the front prop shaft (out of balance), which turns when in 4-auto, but not in 2WD.
I have two Ram's and when I first ran them in 4-auto, I did notice some very small differences in vibration (very faint), but the more I ran it that way, it smoothed out to almost not noticeable. In my case, guessing the bearings were breaking in a little. And I am very susceptible to vibration.
I have two Ram's and when I first ran them in 4-auto, I did notice some very small differences in vibration (very faint), but the more I ran it that way, it smoothed out to almost not noticeable. In my case, guessing the bearings were breaking in a little. And I am very susceptible to vibration.