Firebreather

Hi, I wonder if any of you knows what this is. My car gets this awful vibration when I'm cruising in overdrive and push the gas pedal a little further to go faster without stomping on it for a gear change. The same happens when going uphill and just apply a little more gas. It's like if the transmission needed to shift but it doesn't and the car starts vibrating. It's really annoying and I really don't know what could it be? Help please. Thanks