LAShaker

I have been following this thread for well over a year but this is my first post. I have had this dreaded vibration ever since my odometer turned 800 miles. It happens in the 40's and 70+. I drive mainly on the interstate, so I shake most of the time. My truck came with 22's and my dealer has replaced 11 tires and tried different rims with 0 success. They eventually put the 18" rims back on that the truck came with from the factory with Bridgestone AT tires. This reduced the vibes greatly but not totally. They have done the PICO and GM says it is "within spec". So I believe they have done all that they can do. I have driven it this way for about year. Last week I tried something that I had read on another forum. I put the truck in M5 and turned off grade braking. I have been driving it this way for just over a week on the interstate at 70+ speeds. Hardly any vibrations at all. My truck is tuned with an Intune so I've had AFM turned off for a while. My question is, what would cause the vibrations to disappear by driving in M5 with grade braking off compared to driving in D with grade braking on?