Combo of things. Axle design, Suspenion system be it leaf, coil or a combo of types, to name a couple of the bigger one's. It all comes back to the 1st one, and the work order for the Gov't to have a auto that did abc. Thus you got the jeep, and even though the look has changed over the years, the vision of what it is and how it acts has stayed the same for the most part, with flex being just one thing to consider.