For some reason my 'burb has developed a shake in the seats above 60 MPH. The steering wheel stays true, but the seats (and the rest of the back) get a noticeable shimmy when I get above 60 MPH. It is more noticeable some times than others. It is almost as if it will catch an irregularity in the road and then amplify the shimmy. I have a 6" RCD lift kit, and 35" Nitto Terra Grapplers. When I put the kit on I took the driveshaft to a local shop and had them lengthen the shaft (can't recall how much, but my mechanic measured it and told me at the time how much longer it needed to be). This is a 2003 2500 suburban 4x4. I have had the tires rotated and balanced twice, and that did not solve the problem. I got underneath and tried to wiggle the rear driveshaft, but it is snug as can be so i am guessing it isn't slop in the u-joints. The kit has about 40,000 miles on it, and the truck has about 46,000. I would guess the vibration started about 5,000 miles ago. Maybe a little longer. Anyone know how to make this annoying shake stop? Thanks.