I just finished a 400+ mile trip to Maine on my 4.8is, and yes at times it does feel the car is tramlining, my solution is to allways drive on the left side where most of the heavy trucks would not drive. Also I would check if the different tread pattern would cause the issue (like other members have suggested).
On a side note, this was my first long trip as well and absolutely loved the engine growl and the effortless drive....btw I was surprised how good it was on the trails with the sports tires cause my buddy had a pathfinder and I kept tellin him that i'd be stuck since x5 is basically an on road car, but to my surprise it did'nt plus the Hill Descent control proved great where even the pathfinder felt a lil out of control (as per him)