A few things come to mind:
1/ Pinion angle - If you have aftermarket shafts and the front pinion angle is not good you'll have a vibration and one of the places you'll feel it is in the gas pedal, usually around 50-55 mph when you are on the gas.
2/ Exhaust - If the manifold or manifold-pipe bolts are loose then this can also create a vibration noticeably through the gas pedal. If you have aftermarket skids, there may be contact on the exhaust cross pipe, again more noticeable on acceleration.
3/ Engine vibration - The engine may tend to vibrate a little under certain loads, again this is usually only noticed through the gas pedal. Next time it happens, change gear or hit the OD button and see if the vibration changes (or goes away) with different RPM.
Not sure why these things are felt through the gas pedal more, but I'm guessing it's because the gas pedal is connected to a large block on the firewall and that picks up the vibration.