1999 Ford Ranger Rough Idle

I started my truck today after 4 hours having driven it 49 miles - and when it started, it idled very rough in reverse and drive. When I put it in neutral or park, it ran smooth(er). Upon acceleration, the truck felt like I was driving on a bumpy road and my check engine light came on. I have been having trouble with my gas cap
(locking version) staying locked or "on" (don't know if that means anything).