I had a truck that started doing that, not a GMC but... anyway in my case there was a steering dampener on the tie rod, looked like a shock absorber. It had leaked out all it's fluid. Replaced it, back to normal just the occaisional wobble until it dampened out. Eventually put new tires on it and the problem went away completely.
Don't know if your truck has one or not but I would start there and then maybe have a tire shop try spin balancing your front tires?