My van is idling rough and cuts off while I'm driving, what could cause that?

Your description sounds typically of what happens with a dirty throttle body. . A dirty throttle body will adversely effect idle. Combustion residues build up in the throttle bore and throttle plate thus limiting critical air flow past the throttle plate when closed. The Idle speed motor cannot compensate fast enough and the engine stalls. To clean, Remove the air cleaner hose, Fully Open throttle plate (Engine off), spray a small amount of Throttle Body Cleaner in bore and wipe with shop rag until bore and throttle plates are clean. Other possibilities are intermittent Crankshaft Position or Camshaft Position Sensors