Possibly Imperial Leviathans, converted to civilian use.
I have to reread the fluff, as I havent hit this thread in quite some time, but maybe the colonists had an STC or a Manufactorum ship that produces Leviathans. They are used as clan vehicles and wander the wastes of the planet, meeting once a year for social gatherings, recruitment by the Emperors "Star Warriors", and clan marriages.
Leviathan not Sandcrawler. GW not Lucas.