1999 Mercury Mountaineer • 140,000 miles

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?
October 30, 2012.

Have your rear driveshaft ujoints checked, they may be worn and that is where vibration for that type of thing is.

Oct 30, 2012.