Tires: Vibration/bouncing, continental contitrac, vibration problem

Question
I recently purchased a 2006 Chevy crewcab and noticed a vibration and bouncing issue at least compared to my previous crewcabs.  I took it in for warranty repair and was told nothing was wrong with the shocks/etc.  So my question is could it be the tires?  They still have plenty of tread and I had them rotated and balanced and that didnt help.  I noticed that the tire sticker on my door shows OEM of 245/70 R17 and the tires on it right now are Continental Contitrac 255/65 R17.  Should I try replacing the tires since I was told nothing is wrong with the shocks and if so do i go back to OEM size?

Answer
Daniel,

There are a lot of reasons a vehicle vibrates.  Rotating tires with no change usually eliminates tires as the issue.

BTW, I am aware there is a vibration problem with Chevy Crewcabs, that basically can't be fixed.  If comes from the center driveshaft bearing.  Maybe by this time they've figured out what to do to fix it.