|The following is based on Fallout Tactics and some details might contradict canon.|
The design is based loosely on the M1025 Hummer as built by AMG in the late 20th and early 21st century. Many of the parts used are original and a testament to the quality of the original vehicles. Used by the Brotherhood of Steel as a general purpose work horse, it's a fast and reliable way to get about. Well armored against handguns. The Warrior receives a Hummer during his mission in Macomb.
How the Brotherhood acquired such a large fleet of these vehicles remains a mystery.
The Hummer appears only in Fallout Tactics.