Thanks for the note, I should have been more clear. I'm used to less traction when installing new tires, and this is the second set I've put on the back of the M. And the behavior was markedly different than all previous experiences.
Interestingly, on that link you sent was a guy describing a similar concern to mine. Maybe it's just a) in cold weather with the A/C off there's more power going to tires with less grip, and b) it seemed worst on roads that have some traffic depressions (i.e. tramlining). Maybe when the tires are new and the shoulders aren't worn at all, that makes more of a difference?