Check Engine Light

I'm experiencing an intermittent problem with my '96, C-3500, 2wd,5-spd,5.7ltr. During long drives the check engine light comes on. The truck still drives fine, no loss of power, etc. The light doesn't stay on after shutting the truck down and letting it sit overnight. Following morning all is well. No problem during short trips. Just long highway trips. Anybody have any thoughts as to what it might be?
Thanks

Take it to AutoZone and they will check the codes for free.

Thanks! I'll try that.