I have an issue that just recently started with my 2005 z71 Silverado. It only happens given certain conditions but I'll explain what I do to replicate the problem. When traveling about 50mph, light throttle to maintain speed the truck will begin to violently shake/shudder unless I: Tap the brake Hit the tow/haul mode Hit the gas Drop to neutral If I'm traveling below 45 or above 70 it doesn't happen. If I'm accelerating through the range it doesn't happen. 50 seems to be the sweet spot but it does occasionally occur anywhere in that range. I topped off the transmission fluid which did ever so slightly lessen the effect. Other than that I was going to start with a fluid change, and maybe replace the converter, but figured I'd float the question here first.