i have done a search and looked in my manual. i'm just not sure when to use it. I can shut it off in my TJ but when should i? should i only turn it off if i'm hauling something? this is my first jeep and my first car with an overdrive, or the option of turning it off.

I only turn off an overdrive if the vehicle is shifting from drive to overdrive too much.

how do i know when that is happening?

If you see your tachometer go up in RPM or feel the vehicle downshift it is happening.

Reading the owners manual may be of some help.