Check your motor and transmission mounts. My grand caravan (same engine and transmission) had that problem, as did my voyager at one time (then again the voyager was known to spontaneously throw axles all over the road too, but that again was a different story

!. Anywho, have your motormounts and transmission mounts checked....it sounds like one is loose or nonexsistent....usually that "clunk" sound is the sound of the transmission engaging and bumping into metal, usually the subframe. Those mounts being loose can also cause irratic shifting, as well as cause the car to not shift at all, and ruin axles or the entire transmission. In my caravan, it wouldnt shift into third, until coming to a complete stop, shifting into "park", and then shifting back into drive again and taking off. The voyager had a bad habit of "skipping second", where it would shift into second only briefly, and suddenly jerk into third, and sometimes even fourth, and at normal highway speeds with the cruise set would suddenly jump into passing gear. Ironically, these problems never will cause a service engine soon light or something similar to go off (even when the go off when u sneeze or such usually), since there isnt one that pertains to the tranny...how stupid

....go figure. Thats my