97 Astro gear shift issue

When my ASTRO has been driven for 45 minutes or so, it jerks whenit shifts from 1st to 2nd and is fine after 2nd. It is an Automatic transmition. Also, if I put the van in D2, it does not have the shift issue. Any Clues....Please!

If the service engine light is not on i.e. no code, it's a physical problem with the trans. The pressure is building too slow from first to second. could be a sticking valve, could be a bad seal, could be the valve body is warping.
On another note have you checked the trans fluid level lately? Do this with the engine warm and running. Filter might be clogged up as well.