I don't know the specifics but I was at a Hummer happening with guy who did the same thing. He did regear and said it did fine. With regearing the speedometer was still accurate he said. He also did very good with it off road. I believe he had a 9" lift with 40's. He did get bigger offset rims so the base would be wider to handle the added height.
www.h2ohummers.com