1990 f-150 tranny and engine problems, help!

I recently bought a 1990 f-150 with high miles. (5.0l engine/14 bolt pattern automatic overdrive tranny) I got it running decently but the tranny slips. It seems like it's fine through the first few gears but when you try to accelerate in overdrive it slips. Can I get a band tighened or is it wear on the tranny. I just changed the tranny filter and added lucas stop slip to the tranny fluid without alot of success.
Also, the check engine light comes on after I drive it for about 10 minutes. I know there's a way to pull codes off of it without a reader cause of it's age. Can someone please tell me how. I tried searching without much success.
Thanks alot,
-jacob