94 s420 driveline vibration

my 94 s420 has had a driveline vibration since my purchase @ 72 k miles. new set of michelins (proper speed rated), both driveshaft cushins, trans mount, driveshaft phasing have made no change. i would describe this vibration as high frequency. it starts about 42 mph, becomes very bad near 53 mph and fades the faster you go. never been over 95mph. help!!!!!! this vehicle now has 105000 miles and problem has become NO WORSE.