solved my AWD vibration issue
I know this is an old thread, but I had similar vibration problem in AWD from about 15 to 30 MPG. Recently, I had to take my Envoy to a new shop for a new water pump, and they discovered an outer tied rod end was bad. I had it replaced, but not only that, I replaced it with the heavy-duty option that was available. The part cost only a few more bucks. And since I lifted this truck almost 4 years ago, I replace everything with heavy-duty, where possible.
Shortly after I got it out of the shop, it rained. I flipped it into AWD, and shocked -- first time ever since I've owned this vehicle (even before the lift): no wobble/vibration at take off.
The old shop I used to take it to had looked at this before and not found any issue. But the new shop detected it. So if you're having the issue, I suggest a second opinion on the outer tied rod ends, and even if you end up replacing them under normal maintenance, I'd recommend the heavy-duty part option.
Best wishes, Everyone