It's you're Anti-Spin, mine has done it since new when it's wet outside, and slightly turning, making a U-Turn, giving a little throttle and it's wet outside. I've never really noticed it when it's dry, but I can make it do it pretty much on command when it's wet.
In low- modderate throttle conditions, where the a loss of traction is present for a momment, you're shudder is both tires trying to lay down grip at the same time. This translates into a slight vibration or bounce and quite frankly is you're anti-spin telling you it's working. If you're truck doesn't do this in those conditions, you likely have an open type rear-end, check you're clove boxes to be absolutely sure. I driven a dozen of these CTD's brand new & used and they all have done it.
-Ryan