This is going to sound a little crazy, but bear with me. I have a 98 Yukon and it used to do that as well. It was the worst when I was visiting a friend and had to drive on a long, winding mountain road. I also noticed it only did it when I had a few people in the car and was accelerating up to highway speed. I had the trans shop check it out and they found nothing. As routine maintenance, I changed the fuel filter and the shake stopped. Turns out if the engine started shaking a little from starvation, the converter wouldn't engage. Now, I change that filter twice a year. Ever seen how small that thing is? I also put a lot of miles on mine, so I run a lot of fuel through it.