Ford Ranger vibration at 60-80 mph

I have a 2006 ford ranger Fx4 4.0, for the past few months i have noticed for the past few months that around 60-80 mph my truck will seem to shake or vibrate. of course my first guess was u joints so i replaced and it didnt correct the problem. Another thing i noticed is that it isnt as bad when steady on the gas but once i let off it worsens until i put my foot back on the gas. any guesses?