It would probably be the National Highway Traffic Safety Administration (NHTSA)
Go to:
www.nhtsa.dot.gov/cars/problems
Scroll down to the Technical Service Bulletins link. Enter...Year-2003, Manufacturer-AM General, Model-look under both "H2" and "Hummer"
Interestingly, if you check under the Consumer Complaints link here, you will find one dated 10/24/2002 about an H2 losing its front wheel.
Happy hunting...