I have a 95 Mark Viii with 110,000 miles and I can't seem to get rid of the vibration it has. I notice the vibration around 70 mph and the vibration and rumbling sound increases with speed. This vibration problem started around 80,000 miles. The first thing done was new tires, which made no difference. Then replace the torque converter, still no change. Then we notice the extension house on the transmission had loosen up, so we tighten that and it change the nature of the vibration. Then I decided to buy a cobra 3.73 rear differential from Maxx at Five Star for performance reasons. I also got a MM drive shaft from Denny's drive shaft. After changing these parts I still had the vibration, but then the vibration and rumbling started at 45 mph. Then I decided to change the extension housing on the transmission with a new one and replace the rear wheel bearings, this time the vibration and rumbling didn't start until 65mph. The last thing I did is replace the whole transmission, I found one out of a 97 Mark with 40,000. After replacing the transmission I still didn't notice any changes in the vibration or rumbling sound. The vibration and rumbling seems to be coming from the middle / rear of car. I can't think of anything else to check and the Lincoln dealership has looked at the car and they have no clue. FYI the car has not been in any kind of accident.

crank pully not working maybe , try replaceing the harmonic balancer. other wis i would have to drive the car. to be exact.

and please post times and pics, i want to see that monster, it sounds nice.