Lugnut I have an '05 Silverado like yours but mine has a clunk between 1st and 2nd shift. It's like both driveshafts have to wind up. Dealer says it's ok.

 

No, mine doesn't do that. All shifts are smooth and quiet.

However, it will shift hard with sort of a clunk if the Tow/Haul mode is engaged, I'm driving slowly, and there is no payload. With no payload, I engage the Tow/Haul mode in stop 'n' go traffic and when driving on very tight mountain roads because it keeps the trans from constantly upshifting and downshifting. The downside being the rather stiff shifts when it does shift. Again, this is with no payload (Incidently, the manual states that there is no advantage or disadvantage to using the Tow/Haul mode in this way. I disagree, but the important thing is that no damage to the trans will occur.).

Anyhow, could yours possibly be stuck in Tow/Haul mode? There is a dramatic difference in the shift points and shift stiffness when it's engaged. Have you used this feature at all? If not, engage it and drive around a parking lot. If there's no difference in the shifting, then it may be stuck on.

 

I'll try that, Thanks. Mine will go into 2nd gear at about 15 mph with no payload. The tow/haul mode will shorten the oil change interval according to the computer but it will be worth it to keep it from clumkig.