I have not a lot experience in driving a truck or even mechanic and I have just recently bought my Kenworth w900L (AG 460, 18 SPD, Cummins ISX 600 HP) of year 2006 from local car dealership, and actually I didn't expected so many problems at the beginning, because I had many conversations and discussion with mechanics before purchase. I have sometimes transmission malfunction and when shifting from high speed. Also it is hard down shifting sometimes with strange loud noise and vibration. Maybe drive a truck is just not mine, who knows, but I think such problems with new truck is abnormal.

You don't have much driving experience and you not only bought a truck but you bought it from a CAR dealer that probably knows less about than you do! What could go wrong?
You need to get that truck to someone that KNOWS what they're doing, if not a good mechanic at least a driver that knows how to drive and what an 18 spd should 'feel' like.
BTW a 2006 is not "new" That truck could have been on the road for 10 years now and have well over a MILLION MILES!