"I had a similar issue but with a Ford, actually a couple of Fords (all LB CC DRW) It was finally determined to be just that driveline shudder / driveline wrap, most likely due to the carrier bearing being "soft" and allowing the driveline to whip around, as CC, LB has a long driveline.
Were all your trucks the same? CC/LB..ETC?
Do you know any Ford or Dodge owners? Perhaps a test tow with a non GM is in order."
They have all been crew cabs but both long and standard beds. Though it could be the driveline in all of them. Unfortunately I don't have any friends with hitches on their trucks that I could test out.
I've read a good bit on the issue and know that some ford diesels had a similar issue due to accelerator pedal input or something similar. I thought it may have been that issue since they're all drive by wire now but it does it even when using cruise control which elliminates the possibility of my foot bouncing on the throttle.
