1999 Chevy Astro bumps or jerks hard when shifting to 2nd s

My Astro van shifts hard from 1-2 somtimes and not quite as hard from 2-3. It only happens after I've been driving on the highway over a certain amount of time. It does quite fine when driving around town. Any ideas on whats causing the problem? I don't have the money to replace or have it rebuilt and am hoping it could possibly be an at home fix. Service engine light came on and autozone gave me the code P1870 I believe. Just says transmission is slipping. Very general explanation. Please help