I don't think wheeled vehicles make much sense in fallout, but what would work well on the rough terrain would be a horse. As there aren't any horses around, why not make one. In fallout 3 there were the poster ads for Giddy-Up Butter cup, a robotic horse for little girls, in the Mother Ship Zeta DLC you could see a large number of them, they were about pony sized; ever since then I've been thinking how cool it would be if there were advlt sized Giddy-Up Buttercups. They could be powered by fusion cores to make them both more realistic and a resource allocation question (suit or horse until you have enough that you don't care). You could even mod them to have different cosmetic changes or utility like better carry weight and use them as additional storage when carrying heavy loads.