What is the correct drive cycle for ODB in 98 Chev Astro Van?

You're asking about an ODB drive cycle but you need to know that your model year vehicle is actually what is called OBDII: On Board Diagnostics, Version 2.

Generally, what is called a "drive cycle" is somewhat different from country to country but refers to various data points used to determine the operating functionality of a given vehicle.
There are pre-qualification parameters that must be met before the readiness monitors can begin testing. Some of those parameters are battery voltage, fuel level, etc.
Many require the Engine Control Module (ECM) begin with a cold engine start-up, operated at a certain speed for a period after the reaching operating temperature, and then, for some of the readiness monitors, cool to below a specified temperature.

Here’s an example of a drive cycle readiness routine for a Chevrolet Trailblazer.

At least 17 hours have elapsed since the last drive cycle met and passed the test criteria. That means you can’t clear codes and immediately try to perform a drive cycle and zoom off to the testing station. Remember, minimum of 17 hours.

• Check engine light is off.
  
• No Emission related trouble codes are present.
  
• BARO signal more than 74 kPa.
  
• Battery voltage between 10-18v.
  
• Difference between the ECT and IAT signals less than 14ºF at startup. In other words, the two sensors must be good.
  
• Engine coolant temp between 39-86ºF at startup. Can’t start this test on a sub-zero day.
  
• Intake air temp signal between 39-86ºF at startup.
  
• Fuel level between 25-75%.

If you meet all those criteria, then

1. Start the engine and allow it run at idle speed for 15 seconds.
2. Accelerate at part-throttle to 45 mph and maintain that speed until the engine reaches normal operating temperature (this can take up to 10 minutes).
3. Drive for another 3 minutes at 45 mph on a flat road without climbing any hills to minimize the fuel slosh. Bring the vehicle to a stop in a safe place, but do not turn off the
Ignition key.
4. On the scan tool or code reader with readiness monitor feature, check the status of the readiness code for the EVAP System Monitor. The Readiness code should change to
YES when the test is done. If it does, return home turn off the ignition key.

If it does not, check for any temporary codes. If no temporary codes are set, return to step 3 and repeat the test procedure. Check the status of the Readiness code for the EVAP System Monitor. The Readiness code should change to YES when the test is done.

At this point (if you're still with me... ), you can see it's not a simple thing.
This may be why the term "technician" is becoming preferred over "mechanic" when referring to they guys that do drivability diagnostics.
I hope this helps a little.