Check the vacuum hose attached to the throttle actuator. It may be damaged due to heat from the exhaust manifold. The hose runs from the rear of the engine along the driver's side to the front by the battery.
Check the forums at http://www.astrosafari.com/ for solutions for this
Is the vehicle speed sensor working? If it is not, the service engine soon light should come on when driving also. The speed sensor is located behind the instrument cluster, where the speedometer cable attaches, on the older vans, and I think it might be on the transmission on newer ones.
If the van is older, the wires from the cruise control switch may be shorting out in the steering column.
Another thing to check is the brake switch. Above the brake pedal there should be two switches, one for the brake lights, and one for the cruise control. They are probably both built into the same unit, or at least are very close together.
Finally, the cruise control unit itself is a box attached to the cowling near the windshield washer motor. You can check the cable that comes out of it and goes over to the throttle.