I have a 2003 Trailblazer. I normally run it in automatic 4WD. I've noticed recently that I get a strange hiccup, usually when cruising at 55MPH down my country road (the same road I drive day in and day out). At first I thought it was an engine miss, but did the tune up and I'm still having the problem. However, I drove this morning with the vehicle in 2WD and did not experience this hiccup. I'm beginning to wonder if there's an issue with my automatic 4WD, hitting bumps that engage the 4WD for a split second. As far as I'm aware (last time I used it) all functions of the 4WD work (I just don't have any indicator lights on the switch, though the N works because in the dark I see it light up when I first start the engine). There is no 4WD service light on my dash. This is a seemingly random issue that's driving me crazy! Anyone have any ideas?