Sounds like the camshaft sensor; it would be worth it to replace the camshaft sensor and the crankshaft sensor... It is VERY much a pain in the neck to change the crankshaft sensor, so change the camshaft sensor first; actually they call it an inductive pickup... It's that black, round, plastic thing UNDER the distributor cap...
Your timing chain may be VERY slacked, making the camshaft sensor unable to compensate... The timing chains last a lot longer in the newer vans and trucks, and being that it seems like it just started running bad, "all of the sudden"... I'd try that "inductive pickup" first...
Hope that helps...
Mike