When I go over bumps, I hear a rattling noise like something is banging coming from around the tranny/transfer case. If I put my hand on the 4x4 shifter when this happens, I can feel movement/vibration with the noise. The noise only happens when I am coasting or breaking; it does not happen when I am on the gas. I also hear a very similiar noise when I hammer it and the transmission shifts down hard. Unless there is something loose inside the tranny, I suspect the cause might be the transmission mount going out. The mount looks ok by casual inspection, but is there any way to test the transmission mount to see if it has too much play in it before I go and replace it for nothing?

My truck is a 2000 F150 XLT 5.4L 4x4 Automatic Transmission