Most likely the brake booster is your trouble! Next time this happens, pull the vacuum check valve out of the booster then put it back in place, now try to move van, brakes free and not locked now? Get some help if unsure how to do this. Do NOT drive with check valve removed from booster!!
The rubber brake lines from body to brake cylinders will detereriate on the inside and block fluid flow. If you can determine which wheel you can have it checked. You may need to change this hose. Usually the wheel will be hot after driving.