I towed about 15K this weekend this the the most I have had on this truck. The truck handled it great but I noticed when I take off it feels like it is shuddering. For lack of better way to explain it. Its very obvious and only does it in first gear. As soon as it goes to second it goes away. I also don't notice it unloaded. I didn't notice it at 10K either. Is this normal due to the large load or do I need to keep an eye on something or take it in to get checked out?

Could be spring wind up

Several of guys have been talking about this exact problem with the Medium Duty Kodiaks. Several guys have taken to dealer for diagnosis and repairs. I've heard that there is a service bulletin for the Kodiaks with this issue that are equipped with factory exhaust brakes. I'm paraphrasing, of course, but if memory serves there is an issue with the transmission plates not fully engauging. Check out Medium Duty and the thread Trans Shutter.

I will say from experience that what you are describing is neither normal nor mechanically sound. At minimum get your dealer to evaluate and create a paper trail.

Thanks Knighte that sounds exactly like what is happening to me. Guess its time to stop by dealer and see if TSB is in effect for 2006 as well. One question that I wonder is why do they put in the two piece drive shaft if this is a problem? Seems with GM cost cutting it would be cheaper to install a one piece drive shaft from the factory rather than the two piece with carrier bearing. Guess what I am wondering is what is the advantage of the two piece shaft or what am I giving up by having them install the one piece shaft?

My truck did the same thing for a while and I did not think much of it until it just got worse. I was also feeling it upon take off and only in first gear while towing a 15k fifth wheel. This weekend while towing a 9k tagalong trailer the shudder became much worse. Now it happens both while taking off and stopping, both right around 10mph. Now I also feel it when not towing. Truck is at dealer and they say that they have no idea, need more time to diagnose.

When I started to tow my 5th wheel I noticed the same 'shudder'. I found a TSB for my truck addressing this problem and it was due to the 2-piece shaft. I replaced the carrier bearing assembly because it was destroyed. Within 1 month I destroyed the new one. I finally found a guy off of Ebay that builds aluminum driveshafts. I gave him the OEM Spicer part number and he was able to build me an OEM shaft. No more problems since then!