1997 Explorer Shuttering/Chugging
 

My wife's 97 explorer has a weird issue. It's a v8 and when we drive around at certain speeds/rpms it just starts to shutter and chug. If i give it more gas or push the O/D off button it gets better. Is this a tranny problem (hope not) or fuel pump, fuel injectors, misfiring, spark plugs, water pump, what? I dont have a clue where to start but I dont want to just buy/replace stuff unless I know exactly what will fix it. Thank you to anyone for your help. :)