My 2001 Subrurban just started this too! Mine is so bad that it feels like the wheels are falling off! My wfe was driving our other car in front of me the other day and we were on the highway at about 75mph. I came to a bridge(the small bump where the highway and the bridges pavement meet) and this thing shook so bad that she saw both front wheels bouncing all over the place from her rearview mirror and the car next to me even slowed down to get away from me...thats how bad it shoook. I immediately begain slowing down and as soon as I slowed to about 50mph it was fine. This has happened a few times now. My truck is completely stock. I had a shop do an alignment and they said it was out a bit but the outside edges of my 1.5 yr old front tires are completely rounded over.