[3Hummer]

First off welcome to the forums, welcome to the Hummer world (if it's your first) as is tradition here, we want a picture! or two or three or you get the point... But to answer your question I had a problem on my H3 Alpha that was causing a vibration (can't remember at what speeds). Turned out to be one of my skid plates under my truck was slightly loose, at certain speeds the vibration would get to a point that you could feel it through through the steering column. I couldn't figure out what it was until one day I kicked side step to get some sand off my shoe, when I did that I heard a non normal vibration/rattling sound, I was able to then follow that and found the plate that was loose. Tightened it up then no more problem! I tried getting under the truck when it was on and was never able to hear it, I guess over the noise of the engine I couldn't hear that rattling, and I didn't even think to look at a skid plate at first. Not saying this is your problem (but it could be), but make sure to even check the simple things. On the more complex side of things I had another similar problem with my first H3 couldn't figure out what it was but it occurred at highway speeds, eventually the transmission gave out and had to be replaced sheered gears and everything, it was the automatic so not my doing! and luckily it was all covered under warranty, but once that was replaced I never had the vibration problem again in that H3, hopefully its not something that serious but with a truck with so many moving parts theres so much that it could be. I hope you find the root of the problem and its something easy and cheap to fix! Once again welcome!