My mountaineer has a vibration under the pedal through the floorboards, it only happens once I hit about 58 mph. it gets louder the faster I go, until i get to about 65 mph. then it quits down a little but does'nt go away until I slow back under 58 mph. I checked and filled the gear lube in the front diff. It was low because I have a slow leak through th rear seal. Does'nt leak bad just about half pint in over a couple years. My driveshaft looks straight and can't see any wieghts missing. and now about one week after it started there is a slight clunk like a cracking kind of noise and you can feel it as you drive. Does anyone have any ideas?